I don't think the Openreach ONT is going away for BT FTTP customers any time soon, so for now you are free to just plug whatever router you want into your ONT.
If you're the kind of person who cares about your router, why would you care about their proprietary VoIP service? Just use any other VoIP provider of your choice.
This was 2016. The battery box was there to provide power and service for a landline in a power cut.
My router plugged into that RJ45 with a pppoe client, username "bthomehub@btbroadband.com" and looks like any old password ("BTSux" for example)
If you want to have a functioning "landline", why would you choose to use one bundled with your internet provider compared to a much more competitive independent voip provider?
This is a good summary of the decline: https://www.wrappz.com/blog/decline-of-the-landline/
So much this. Even the local Cable co. charges more for residential home phone than competitors like Ooma do, and of course if I wanted to go the whole way with deploying small SIP PBX in my house I could do it for less than a couple bucks a month getting a DID from Flowroute.
I guess getting it bundled by your triple-play provider of choice means you don't have to worry separately about battery backup because regulations usually require they provide that with their CPE; but it's not a big deal to hook your equipment up to a UPS either.
To the other questions in the UK yes you can move your landline telephone number to a third party open standard sipp voip service, but setting up a voip adapter to pretend to be a landline is fiddly for many and migrating a landline telephone number, while it can be done, is not a nice process in practice. If you have an openreach ont with a telephone socket built in don't expect it to do anything in the future. The industry went another way. If you wish to carry on plugging in a telephone to your landline without any configuration then you will have to start using the telephone socket on your wifi router and only the router provided by your isp. (With some exceptions). Though most people won't care because they just use their mobiles now...
I find the need to configure a router or VoIP adapter to be a strange, over-engineered concept when it comes to replacing POTS. We're already authenticated by virtue of being physically connected. The exchange should be able to pass through the identity of the connecting line and no authentication or manual configuration is a fundamental requirement. In PPP, authentication is optional, but BT/OpenReach require it and complicate everything for consumers for no good reason. Since nearly every line has only one provider, they should keep track of that at their end, and then routers wouldn't need PPP configuration in the common case. Everything could be negotiated automatically, and the protocol already supports this!
We do have TR069 but that adds even more unnecessary complexity.
The same goes for a POTS replacement. Authentication is not fundamentally necessary. They could autodiscover, and then the identity of your physical line could be passed through. There isn't an obvious protocol here, but it's trivial to achieve technically as long as it isn't overengineered (see for example uPnP IGD vs. NAT-PMP). If this is a real problem, it can be addressed.
I don't think it's part of most consumer's threat models that it matters if their line identity is intercepted and used by an adversary, since we all use higher level protocols to establish higher level authentication anyway. But if it were, then TOFU together with an out-of-band update mechanism (eg. "call customer service to activate your new phone and/or router" or just "scan the QR code on the side of your phone and/orrouter with our app to activate it") would be all that's needed to deal with that. Client side authentication still wouldn't be needed, and can't address that threat model directly anyway.