3CX is software, and software can't make a phone call on its own. It needs SIP trunks (the connections that carry your calls over the internet), and the provider behind them decides how clear your calls sound and whether your phones keep working when something goes wrong.
With most phone providers you never think about this, because the lines come bundled in and nobody mentions them. 3CX is different. The licence gets installed on a server, either your own or one in a data centre, and then someone has to connect it to SIP trunks before a single call can go in or out.
So someone has to pick the SIP trunk provider. It's easy to treat that as an admin job, but we think it's one of the biggest decisions in the whole set-up.
Why picking a SIP trunk provider is harder than it looks
3CX works with almost any SIP trunk provider, and there are hundreds of them. Having that choice is useful, but if you're setting up your own phone system you've got very little to judge them on. Most businesses end up going on price or picking a name from a list, with no real way to tell a well-run network from a badly run one.
You find out later. A weak provider means patchy call quality and service that drops in and out. And when something does go wrong, it decides how long it takes anyone to work out what's happened and get it fixed.
We learned this the hard way. Over the years we've had providers make changes to their networks outside agreed maintenance windows. One provider we used for some services had its whole SIP network knocked offline by a DDoS attack (a flood of traffic designed to overwhelm a network).
In voice, stability is everything. These days we judge a carrier on how well it looks after its network and what disaster recovery it has in place before we look at price.
How we connect every Zen phone system
SIP trunks come as part of our hosted 3CX service, so you don't have to choose a carrier yourself. We've already done that, using what we've learned from more than 18 years of running business phone systems.
Where your calls normally go
Your main trunks come from an established UK carrier. It connects to our hosted 3CX servers through more than one session border controller (the equipment that manages call traffic between their network and ours), so if one fails, calls move to another on their own.
What happens if that carrier goes down
Every system we set up is also connected to a second carrier, and each outbound call rule has a main route and a backup. If the main carrier went down completely, outgoing calls would switch to the second one without anyone having to step in.
The main carrier has its own failover, so the backup rarely gets used. When it does, you won't notice.
The second carrier earns its keep day to day too. Its international rates are better, so calls abroad go through it by default. In some countries it can also pass your calls on inside that country as local calls.
What's included in your calls
On a three-year term, calls to UK local, national and mobile numbers are included. Like any provider offering this, we have a fair use policy, and it's based on the number of channels on your trunks (how many calls can happen at the same time). We size those channels with you, and we can't think of a customer who's ever hit the limit. Calls to anywhere else, including international numbers, are charged.
Example
If you had 60 staff across two UK sites and a sales team that calls Europe, your UK calls would sit within the inclusive allowance. The calls to Europe would go out over the second carrier at its international rates.
Why the ISDN switch-off makes this more pressing
The PSTN switch-off (the end of the UK's old copper phone network, which moves all calls onto the internet) is due to complete on 31 January 2027, and Openreach has said the date won't be delayed again.
ISDN goes on the same day, and for business phone systems we'd say that's the bigger change. ISDN is an older type of digital line that a lot of phone systems still use, particularly in larger businesses with specific calling needs. Openreach stopped selling new ISDN2 and ISDN30 lines in September 2023 and will withdraw them on 31 January 2027. It's also said there's no migration path, so the lines will be ceased from 1 February.
Any business still on ISDN will need SIP trunks or something like them. Plenty will be choosing a provider for the first time, with the deadline not far off.
A question to ask your provider before anything goes wrong
Things go wrong in IT, and more often than most people expect. We work hard to keep that to a minimum, but in our experience you learn the most about a provider from how it responds and how well it sorts the problem out.
If you already run 3CX on SIP trunks you chose yourself, ask your provider what its business continuity plan looks like. Then ask your own business: if your calls stopped this afternoon, what would happen next? We've moved a number of existing 3CX customers over to us, and that question is often where the conversation started.
Talking to us about how your calls are connected
If you'd like a second opinion on how your calls are connected today, book a free consultation with us. We can take a look and tell you honestly what we find, even if that means leaving things as they are.