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If you’re still running your business on a traditional landline or ISDN system, the clock is ticking. BT Openreach has confirmed that the Public Switched Telephone Network (PSTN) and ISDN will be fully switched off on 31 January 2027. That’s not a distant IT problem it’s a firm deadline that affects every business in the UK with a fixed phone line.
Migrating your business phone system to VoIP isn’t just about compliance. Done correctly, it reduces your telephony costs, gives your team greater flexibility, and positions your business for genuinely modern communication. Done badly, it creates downtime, number porting headaches, and frustrated customers.
This guide walks you through every stage from assessing your readiness and choosing the right provider, to porting your numbers safely and avoiding the mistakes that catch most businesses out.
Switch-Off Deadline
SME Migration Time
Cost Per User / Month
Number Porting Time
Cost Saving vs ISDN
VoIP (Voice over Internet Protocol) lets businesses make and receive calls over the internet instead of traditional phone lines. Calls are converted into digital data and delivered through your broadband connection using IP phones, desktop apps, or mobile devices.
For businesses, VoIP reduces line rental costs, supports remote working, and includes features like call forwarding, voicemail-to-email, and auto-attendants without needing a traditional PBX system.
With the UK PSTN and ISDN switch-off approaching in 2027, businesses still relying on copper phone lines will need to move to an internet-based phone system to keep services running.
BT Openreach stopped accepting new PSTN and ISDN orders in September 2023. On 31 January 2027, all remaining analogue and ISDN connections across the UK will cease to support traditional voice services. When planning your migration, treat the published date as fixed and build your timeline backwards from it do not rely on the possibility of regional delays or extensions.
Unless you hold a signed migration agreement with your IT provider or telecoms reseller, the responsibility sits squarely with you. Ask for written confirmation. A surprising number of businesses find out late in the process that their provider had been waiting for them to raise it first.
Important: Ask your IT provider or telecoms reseller for a signed migration agreement in writing. Verbal assurances are not sufficient.
The switch-off reaches beyond handsets. Alarm systems, CCTV, PDQ card terminals, lift emergency phones, and door entry panels connected to analogue lines all require separate solutions ahead of the deadline. Each one needs its own migration plan, so start that audit early.
Most UK SMEs switching from ISDN report line rental savings of between 40 and 60 per cent. Features that previously required significant capital investment call recording, auto-attendants, voicemail-to-email, call analytics are included as standard in most hosted VoIP plans. Staff gain the ability to take and make calls from any device, wherever they are working
Businesses that move early tend to benefit from greater installation capacity, smoother onboarding, and enough time to iron out any issues before deadline pressures start to mount.
There are three main deployment models, and each suits a different type of business. Choosing the wrong one is expensive to undo, so it is worth understanding the differences before shortlisting any providers.
Best for: Most UK SMEs, remote and hybrid teams, businesses replacing ISDN
Your phone system is hosted by the provider instead of an on-site PBX. Calls can be managed through IP phones, desktop apps, or mobile devices over your internet connection. Providers typically handle updates, maintenance, and core infrastructure.
Best for: Businesses keeping an existing PBX system
SIP trunking replaces ISDN lines while allowing you to keep your current compatible PBX hardware. Calls are delivered over broadband instead of traditional phone lines, reducing line rental costs without a full system replacement.
Best for: Businesses already using Microsoft 365 and Teams
Microsoft Teams Phone adds business calling directly into Teams using Calling Plans, Operator Connect, or Direct Routing. It combines voice, video, chat, and collaboration in one platform, though additional licensing may apply depending on setup.
| Factor | Hosted VoIP | SIP Trunking | Teams Phone |
|---|---|---|---|
| On-site hardware | None | Yes — PBX retained | None |
| Management complexity | Low | Medium–High | Medium |
| Best team size | 1–500+ | 20–500+ | 10–500+ |
| Microsoft 365 integration | Varies | Limited | Native |
| Setup time | 1–4 weeks | 2–8 weeks | 2–6 weeks |
| Monthly cost | £8–£25/user | £5–£15/channel | £12–£20/user |
| Remote working | Excellent | Good | Excellent |
Quick recommendation by business type
Under 50 staff, single site → Hosted VoIP
Already on Microsoft 365 → evaluate Teams Phone
Recent PBX under 5 years old → consider SIP trunking
Multi-site or complex setup → Hosted VoIP with centralised management.
Not sure which setup fits your business size, budget, or existing systems?
Easy to miss:
Alarm systems, door entry panels, lift phones, and PDQ machines often run on analogue lines and cannot connect directly to VoIP. You will need analogue telephone adaptors (ATAs) or separate solutions — raise this with every shortlisted provider before signing. Many businesses only discover late in the process that these devices still rely on analogue lines. Auditing them early avoids last-minute disruption.
A quick readiness check now can prevent costly migration problems later
Document every active number (including DDIs, fax lines, and alarm lines), your current provider and contract end date, the make and model of your existing PBX, the number of handsets by location, and your peak concurrent call volumes. Also list every non-telephone device connected to your phone lines card terminals, door panels, lift phones.
Without this audit, providers cannot quote accurately or plan a realistic porting schedule.
VoIP call quality depends more on how your network is configured than on raw broadband speed alone. The factors that matter most are your QoS settings, low and consistent latency and jitter (test using a VoIP-specific tool rather than a generic speed test), and the compatibility of your router and firewall.
If your internet connection is unreliable, resolve that before evaluating providers. If your network needs upgrading router, managed switch, or cabling factor those costs into your budget before comparing provider pricing.
Evaluate two or three providers on criteria that go beyond headline pricing. Poor support and rushed onboarding consistently cost more in staff time and disruption than a slightly higher monthly tariff.
Always ask for a 14 to 30-day trial before committing. Check whether handsets or system configurations are proprietary — these can make switching providers later more costly than you might expect.
Key questions to ask every provider:
Number porting in the UK is regulated by Ofcom under General Condition C7. Your current provider has one working day to accept a valid porting request. Geographic numbers typically complete within 2 to 5 working days; non-geographic numbers (08xx, 03xx) can take 10 to 15 working days.
In practice, the most common source of migration delays is mismatched account details. Even small discrepancies in your business name or address can hold up a porting request. Double-check all your account information against your current provider’s records before submitting anything.
Critical
Never cancel your existing phone contract before porting has completed. If your line is cancelled first, your number is lost. Released numbers are often unrecoverable.
Replicate your existing call setup then take the opportunity to improve it. Configure auto-attendant menus, hunt groups and ring strategies, voicemail and voicemail-to-email, call recording (confirm it is active and compliant before going live), out-of-hours routing, and hold music.
Build and test in a staging environment before switching any live traffic across.
Poor user adoption is one of the most preventable problems in any VoIP migration. Keep training practical: focus on making, receiving, and transferring calls; accessing voicemail; using softphone and mobile apps; and managing presence and do-not-disturb settings.
Short video walkthroughs and a one-page quick-reference card work well for most teams. A 30-minute live session on go-live morning reliably reduces the volume of support requests in the first 48 hours.
Plan the cutover for a low-traffic period Tuesday or Wednesday morning works well for most UK businesses. Before cutting over, confirm:
Most well-planned migrations complete with no disruption visible to customers.
Poor user adoption is one of the most preventable problems in any VoIP migration. Keep training practical: focus on making, receiving, and transferring calls; accessing voicemail; using softphone and mobile apps; and managing presence and do-not-disturb settings.
Short video walkthroughs and a one-page quick-reference card work well for most teams. A 30-minute live session on go-live morning reliably reduces the volume of support requests in the first 48 hours.
The most reliable way to avoid disruption is to avoid a hard cutover where possible. Running old and new systems in parallel lets your team test call quality, routing, voicemail, and mobile apps before switching all live traffic across.
Keep your existing lines active while your VoIP system is configured and tested. Staff can use both during a defined overlap period. This is the single most effective way to catch configuration issues before they affect your customers.
During the number porting window, forward incoming calls from your existing lines to your new VoIP numbers or to mobiles. Callers experience no gap in service.
Move one team or one department first. Resolve any issues at a smaller scale before rolling out company-wide. Businesses that test with a smaller group first usually identify routing or device problems before they affect the wider organisation. This approach works particularly well for businesses with 20 or more staff.
For multi-site businesses, migrate one location at a time rather than switching all sites simultaneously. Each location becomes a proof point before the next goes live.
Agree this in writing with your provider before cutover day. Know exactly what steps to take and who to contact if something goes wrong. A rollback plan you never need to use is far better than needing one you have not prepared.
| One-Off Setup Costs | |
|---|---|
| Provider onboarding fee | £0–£500 |
| IP desk phones (each) | £40–£200 |
| Router / switch upgrade | £150–£800 |
| Professional install (per day) | £350–£700 |
| Number porting (per number) | £0–£15 |
| Ongoing Monthly Costs | |
|---|---|
| Hosted VoIP per user | £8–£25/mo |
| SIP trunks per channel | £5–£15/mo |
| UK landline calls | Usually included |
| UK mobile calls | 1p–3p/min |
| Call recording storage | £0–£10/user/mo |
| Teams Phone add-on | £8–£14/user/mo |
Worked Example — 20-User Office on ISDN30
A 20-user office on ISDN30 typically pays £400–£600 per month in line rental and call costs. The equivalent hosted VoIP system runs £160–£500 per month, with a one-off hardware and setup cost of £1,000–£4,000 depending on handset choices and network requirements. Most businesses in this bracket reach break-even within 6–12 months.
Tip
Ask any provider you shortlist for a like-for-like cost comparison against your current bill. Any provider worth working with should be able to produce this within 24 hours of an initial conversation.
Want a realistic estimate based on your current setup and user count?
| Mistake | Why It Happens | How to Prevent It |
|---|---|---|
| Cancelling existing lines before porting completes | Assuming migration is finished | Never cancel until porting is confirmed in writing |
| Network not assessed before go-live | Focusing on phones, not infrastructure | Run network assessment as step one |
| Choosing on price alone | Attractive headline pricing | Evaluate support quality, SLAs, and porting track record |
| Forgetting analogue devices | Focus only on desk phones | Audit every device connected to your phone lines |
| No staff training plan | Assuming staff will adapt without guidance | Schedule training before go-live, not after |
| No rollback plan defined | Overconfidence in the migration | Define rollback steps before cutover day |
| Accepting provider lock-in | Not reading the contract closely | Check hardware and number portability before signing |
| Ignoring compliance requirements | Assuming old rules still apply | Confirm call recording and GDPR obligations in writing |
Choosing the right VoIP provider involves more than comparing monthly prices. Support quality, porting experience, contract flexibility, and integration options all affect how smoothly your migration runs. Compare providers on these criteria not just headline cost before signing anything.
Compare Phone Systems helps UK businesses compare Hosted VoIP, SIP trunking, and Microsoft Teams Phone providers based on business size, working setup, feature requirements, and budget making it easier to shortlist providers that fit your migration needs before the 2027 switch-off.
Your numbers are fully portable. Ofcom regulations mean you can transfer existing UK numbers to any VoIP provider. Geographic numbers typically port within 2 to 5 working days; non-geographic numbers (08xx, 03xx) can take 10 to 15 working days. Never cancel your existing contract before porting is confirmed as complete — once a number is released, it is often unrecoverable.
A minimum of 10 Mbps download and 5 Mbps upload handles multiple concurrent calls comfortably. Each call typically uses roughly 85–100 Kbps, depending on codec and network setup. More important than raw speed is how your network is configured particularly whether QoS settings prioritise voice traffic. Businesses with high call volumes or inconsistent internet quality should consider a leased line.
A reputable hosted VoIP provider will automatically reroute calls to a mobile number or alternative destination if your connection fails. Confirm your provider’s failover options before signing up, and test them during setup rather than waiting for an actual outage.
Not necessarily. If you have existing IP handsets, they may be compatible with your new provider. If you are replacing a traditional ISDN PBX, you will likely need IP phones or softphone apps on existing computers and mobiles. Many businesses use the migration as an opportunity to go softphone-first, which reduces hardware costs considerably.
Your numbers are fully portable. Ofcom regulations mean you can transfer existing UK numbers to any VoIP provider. Geographic numbers typically port within 2 to 5 working days; non-geographic numbers (08xx, 03xx) can take 10 to 15 working days. Never cancel your existing contract before porting is confirmed as complete — once a number is released, it is often unrecoverable.
Potentially. If your broadband is delivered over a copper ADSL line, it may be affected depending on your area and exchange status. Check with your ISP whether your connection is tied to the PSTN infrastructure and if so, factor a broadband upgrade into your migration plan alongside the phone system.
Yes, when configured correctly. Make sure your provider uses SRTP (Secure Real-time Transport Protocol) for call encryption and TLS for signalling. Change default SIP credentials, restrict international calling by default, set monthly spend caps, and enable anomaly alerts to guard against toll fraud. Most reputable UK hosted VoIP providers manage core security infrastructure at the network level as standard.
Most SMEs with under 50 users on a single site go live within 3 to 6 weeks of signing. Multi-site migrations typically run 8 to 16 weeks. The biggest variable is number porting speed, which partly depends on how promptly your current provider responds to the porting request. Starting early is the single most effective thing you can do to ensure a smooth migration.
VoIP migration is now a practical requirement for any UK business still relying on PSTN or ISDN services. The businesses that handle it successfully are generally the ones that start early, audit every connected device thoroughly, and test properly before cutting over. For most SMEs, a well-planned migration brings greater flexibility, lower line rental costs, and a more scalable communications setup all ahead of the 2027 switch-off deadline.