We're currently with Talk Talk on FTTC and would like to move to FTTP.
Can you keep your landline number on FTTP?
Talktalk's Openreach-based FTTP service doesn't include any voice service. (Presumably the same applies to their Cityfibre and UFO services, but I don't know for sure)
If you install FTTP and simultaneously terminate the FTTC and its linked phone service, you'll lose your number.
If you run FTTP and FTTC in parallel, you then have a number of options for transferring the phone service and cancelling the FTTC. Transferring the phone number to a VOIP provider will trigger the cancellation of your old phone line *and* the FTTC service linked to it.
If you want to go this route, you need to be very clear with Talktalk at the time of ordering FTTP that they should keep your FTTC service running, not cease it. They might also be able to sell you a voice-only service on copper, but it will be expensive.
I've read elsewhere that it's possible that BT won't keep the copper line.
It varies. In some cases they'll keep the copper line with dialtone, and in some cases they'll move you to Digital Voice, where the voice service is provided through the router they supply. You don't get to choose.
Sky provide voice through the router.
Zen will sell you a voice service on their FTTP (presumably through the router as well), although charge extra for it.