Anatomy of a real switch: how a 35-phone office left a dying PBX.

Most articles about “switching your phone system” are written by people who’ve never read a real one’s paperwork. This one is different. We got full sight of a complete procurement record — a UK accountancy firm of about 35 staff moving off an end-of-life Mitel PBX and old BT analogue lines onto a hosted 3CX system, run as a competitive tender between November 2025 and February 2026. Every quote, every due-diligence note, every contract query, the porting paperwork, the cabling schedule.

We obtained the firm’s complete procurement file and read every page. We’ve anonymised the firm and all three suppliers who bid; what’s left is the bare shape of a real switch, start to finish. It’s more useful than any brochure, and a good deal more honest than most case studies — because nothing here is dressed up to sell you. It’s simply what happened.

The trigger: two clocks running out at once

Nobody replaces a working phone system for fun. This firm moved because two things expired together. Their Mitel PBX was end-of-life, with internal notes on it going back to 2009 and a manufacturer security advisory hanging over the older handsets. At the same time, BT served notice that the analogue and ISDN lines underneath it were being withdrawn ahead of the national PSTN switch-off. The old system was dying and the network it sat on was being switched off. That combination is what forces the decision for most UK SMEs, and it’s the one bearing down on roughly half a million business lines still unmigrated as we write this.

Worth noting what they did before committing: they wrote to BT to keep the existing lines running until the new system was live and the numbers had ported. No gap, no gambling on a clean cutover. We’ll come back to why that caution matters.

They shopped like it mattered

Here’s the part worth copying if you’re about to switch. The firm didn’t take the first quote in front of them. They ran a genuine competitive tender — three suppliers, a side-by-side comparison built by hand, and a multi-month back-and-forth before anyone got near a signature. A phone system is a three-to-five-year commitment, and they treated it like one.

It paid off in the most basic way. For the same 35-phone system, the three quotes ranged from about £9,400 to £14,400 over three years. A £5,000 spread on identical requirements is the first and best reason to get more than one quote — a headline number tells you almost nothing until you have something to hold it against.

Two demands ran through the whole process, and both are worth making your own. The first was price certainty: will the supplier fix the fees for the full term, in writing? One bidder wouldn’t, and that alone was enough to drop them — a price you can’t count on in year two isn’t really a price. The second was getting the contract into plain sight before committing, which is where this buyer got genuinely forensic.

The contract forensics: nine questions before a signature

Once the field narrowed, the buyer went through the contracts the way you’d expect an accountant to. One supplier received a formal letter with nine numbered queries before anyone would sign. They’re worth listing, lightly paraphrased, because every one of them is a place a telecoms contract can quietly cost you money:

1. Is this quote scoped for our setup, or is that a generic example?
2. You’ve labelled this “enterprise grade” — confirm whether you’ve quoted 3CX PRO or the dearer AI/Enterprise tier.
3. What are your hourly rates for work outside the contract?
4. Give examples of what counts as a small change versus a chargeable one.
5. Will you fix the fees at this level for the full three years, as your competitors have agreed to?
6. After the fixed term, does it renew annually — not roll into another three-year lock-in?
7. If we restructure the business, will you let us assign the contract, without a charge?
8. Confirm you won’t use our name in your marketing.
9. Your quote mentions inclusive minutes but the contract is silent on them — put them in writing, and confirm there’s no landline-versus-mobile distinction.

Notice question two especially. This buyer spotted the single most common trick in managed 3CX without any help — the “Enterprise” label sitting on top of what is usually a cheaper PRO licence. We wrote a whole post on that move, what an honest 3CX quote looks like line by line, and here was a real buyer catching it cold.

The same forensic eye went over the system sizing. 3CX is priced by simultaneous calls (SC), not by user count, and a 35-person office genuinely runs on about 8 SC. The winning quote licensed it at 24 SC — three times what the firm needed, on a basis most buyers can’t check. Their own staff calling survey, taken during the process, quietly confirmed it: hardly anyone was making the premium-rate or heavy international calls the bigger bundles are sold against. The concurrency was low; the licence was large.

The unglamorous bit: porting, parallel running, and a basement full of cable

A switch isn’t a software purchase. The records show the parts nobody puts in a brochure. The existing numbers had to be ported from BT to the new SIP provider under a signed Letter of Authority, which is the slowest single step in any switch and the one most likely to bite if it’s rushed. Broadband continuity had to be arranged so the office and home-workers stayed connected through the cutover. And in February, somebody spent two days in the basement pulling and testing new structured cabling against a written cable schedule, because the handsets are only as good as the network behind them.

This is why the firm kept its old BT lines alive until the new system was proven. The professional way to switch is to run the new system in parallel on temporary numbers, test it properly, and only then port the real numbers across and stand the old kit down. The cost of doing it the cautious way is small. The cost of a botched cutover — a day where customers can’t reach you — is not.

What this means for your switch

Here’s why all of this matters to you. We’ve taken a real, complete switch apart down to the cable schedule — the bake-off, the contract, the porting, the cabling — and we’ve built how we work around exactly the things this buyer probed for. The detail that catches most people out on their first switch doesn’t catch us out.

• We quote the licence at the SC you actually need, and tell you the number. No 3× over-spec hiding inside a bundle.
• We fix the price for the term, in writing. It was a deciding factor in this tender, and it’s the cheapest commitment in the world to make if you’re being straight.
• Our contract already answers all nine of those questions — the tier stated plainly, an hourly rate on the page, inclusive minutes written in, free assignment if you restructure, your numbers ported away at no exit charge if you ever leave.
• We keep your old lines running until the new system is proven, and we don’t rush the port.
• You own your system and your numbers outright. Because it’s built on 3CX, you’re never stranded with any one provider — the platform is portable by design, and we put that in writing.

If you’re facing the same two clocks — a tired phone system and the PSTN switch-off — the most useful half hour you can spend is letting someone read your setup and your current quote with you. Book a free audit. No commitment, and an honest verdict even if it’s “stay where you are.”