Herr in the UK, Open Reach/BT is switching the old copper cables based PSTN to an all fibre optic based system, and by early 2026 all telephones will need to run over digital fibre rather than analogue copper.
You can read more about the process here:
https://www.bt.com/about/all-ip
https://www.bt.com/broadband/digital-voice
https://www.gov.uk/guidance/uk-transition-from-analogue-to-digital-landlines
It all seems rather unclear what this means to end users and seems like BT are using it to push their own 'Home Hub' router as a 'solution' - as 'Digital Voice' - BT's replacement for analogue 'phones - is proprietary and will only work with Home Hubs.
So you will need a broadband connection of some type at least, or just give up and use cellphones. Tough if you live in some of the wilder parts of the UK.
So how does this affect SMEs like us? Well, like many companies we have an internal PABX (Telephone Exchange) in our office which supports about 30 extensions and a number of BT lines. [Now-a-days we use Zen to supply our telecoms rather than BT, but the technology is the same and uses OpenReach copper lines.] Our exchange works well, and we have no desire to replace it and all the linked analogue extension handsets. Yes, we could rewire the building and lease a package of VOPI handsets and a 'Cloud' based exchange solution, but it's expensive and not what we want.
So, the alternative.. is to keep the PABX, the analogue wiring and handsets and replace the Open Reach PSTN copper lines with a digital SIP running over our fibre broadband. Should just work...
The first thing to look at is a converter box that takes IP in via a RJ45 socket from one of our switches and does some D to A magic [digital to analogue] on it and presents a number of analogue PSTN channels via several RJ11 sockets. At there simplest, these connect to analogue PSTN handsets (good for testing) and, in the greater scheme of things, to the analogue inputs to the existing PABX. It should not be able to tell the difference. We are currently using a Grandstream adaptor which is low cost and 'just works'.
The next part of the equation is to find a suitable SIP trunk supplier, of which there are many. Their offerings are varied and sometimes complex, but we decided to use Andrews & Arnold Ltd. They have been around for 25 years or so and are 'old school' and non the worse for it.
A SIP trunk for a single number incurs a one off fee of less than £2 GBP and a monthly rental of also less than £2. Call are extra (some SIP providers offer a bundle) at less than 2 pence a minute. The minimum contract term is a month, and one can get existing numbers ported in for a one off fee of £15 GBP.
It took us several weeks to figure all this out and determine that it should work and, of course, it does. We were greatly helped by the genial Andrew Calver-Jones at CTi communications Ltd who talked us through every step of the process and was happy to carry on guiding us though the complexities of setting up a VOIP to analogue PABX.
So the end result is a service which will work once BT switch off the analogue PSTN and cost a small fraction of what we pay currently. The downside is a decent UPS (battery backup power supply) is recommended so it all carrys on working if the mains fails!
NB
Voip has it's own language like most technologies and this site is good at explaining some of them - such as FXS and FXO ports.