Switching phone supplier UK: what business owners actually need to know

Switching phone supplier sounds intimidating, especially if you've been with the same provider for a decade and the number on your van is the same one that was on your business cards in 2014. The good news: under UK law you can take that number with you to any UK carrier, the migration is well worn, and most SMEs do it without anyone outside the business noticing.

The honest walkthrough — what to check before you sign with a new provider, how porting actually works, what the legal protections are, and a realistic 5-step migration plan.

First: yes, you keep your numbers

UK number portability is a legal right under Ofcom General Conditions C7. Every UK phone number — geographic 01/02, UK-wide 03, freephone 0800/0808 — can be ported to any other UK licensed carrier. Your existing provider can't refuse, can't charge an exit fee for the port itself (though they can charge for unfinished contract terms), and can't deliberately delay.

The receiving carrier (us, if you join Membership) does the work. You fill in a porting form, we lodge it with the losing provider, and the cut-over happens in a defined window agreed in advance. Callers experience no change.

Due-diligence on the new carrier

Five questions to ask before signing with anyone — including us:

1. Are they a licensed UK carrier or a reseller?

A carrier holds number ranges directly from Ofcom. A reseller buys wholesale from a carrier and rebadges. Resellers can disappear, get cut off in contract disputes, or struggle to port numbers when you eventually leave. Check Ofcom's public Numbering Plan to confirm. Datazero Ltd t/a NumberGroup holds 0800, 03 and UK geographic ranges directly. So do BT, Gamma, Vonage Business, Voxbone and a handful of others.

2. What's the minimum-term commit?

Many UK telcos lock you into 3 or 5 years. That's 60 months of CPI uplift you can't escape. NumberGroup Membership is month-to-month — no minimum commit, no exit fee.

3. Are there per-feature bolt-ons?

IVR menus, call recording, voicemail-to-email, hunt groups, conference bridges — these should all be in the base fee. Anyone pricing them as bolt-ons in 2026 is structurally overcharging.

4. What's the CPI / inflation clause?

BT, Virgin Media Business and many others use CPI + 3.9% annual uplift. Compounding effect — by year 3 of a contract you can be paying 25%+ more than year 1. Read the small print before signing anywhere.

5. Who's on the other end of the phone when something breaks?

Ticket queues, offshore contact centres and 4-hour SLAs are the norm. A small UK team that knows your account is unusual. Worth its weight when your phones go down on a Tuesday morning.

Watch out for contract notice periods

Most UK business telecom contracts have a 30 or 90 day notice requirement to terminate, and many auto-renew if you miss the window. You can still port your numbers out without terminating (the port doesn't require the contract to end), but you may continue to be billed by the old provider until contract expiry.

Read your existing contract, note the next renewal date, and time the switch so you give clean notice. If you're paying significantly more on the old provider than the new one would cost, sometimes it's worth paying out the remaining months anyway — the maths usually still favours switching.

The 5-step migration plan

What an actual UK SME switch looks like:

Step 1: Inventory (week 1)

• List every number you have
• List every line type (ISDN30, ISDN2, analogue, SIP)
• List every PBX feature you actually use
• Note any non-voice equipment on PSTN (alarms, lifts, EPOS — see switch-off guide)
• Check current contract — minimum term, notice period, exit terms

Step 2: Choose the replacement architecture (week 1–2)

On-prem PBX + SIP trunks, hosted cloud PBX, or pure softphone. See the phone-system buyer's guide for which suits which business.

Step 3: Get the new provider live (week 2–3)

• Sign up with the new carrier (with us: join Membership)
• Provision a temporary test number on the new platform
• Set up your hosted PBX or SIP trunk config
• Run a parallel test — make and receive a few calls on the new system

Step 4: Port the numbers (week 3–5)

• Fill in the porting form with the new carrier
• Authorise the port
• Cut-over happens in an agreed window (typically 5–10 working days, sometimes longer for ISDN30 with big DDI blocks)
• For big estates, port in phases — not everything at once

Step 5: Cancel the old service (week 5+)

With port-out complete and the new system stable, formally terminate the old contract in line with its notice requirements.

What can go wrong (and how to avoid it)

• The losing provider rejects the port. Usually because of a mismatched account name or address. Easy fix — the receiving carrier walks you through re-submission.
• The port goes through during a critical period. Don't port during a major sale event or peak season. Plan for a quiet week.
• Forgotten non-voice equipment. Lift phones, alarm signalling, EPOS terminals — anything that quietly dials out on a PSTN line. Audit before port.
• Old contract continues to bill. Numbers ported but contract not terminated. Schedule the termination notice as part of the migration plan.