Would ZEN take over an existing FTTC - that is migrating to ZEN?
In addition would transfer of landline number work with their DigitalVoice setup - probably plug handset into Fritzbox?
I'm aware of new orders likely being FTTP only, however this could be tricky at my location.
If your property has FTTP available(*), and your exchange area is under a "stop sell" for copper, then you cannot migrate an existing FTTC line to a new provider. You cannot even regrade the speed on your existing provider. Any such change will involve installing FTTP and moving you over to that.
If that doesn't apply, then the next thing to look at is: is your FTTC using analogue PSTN, i.e. your phone plugs directly into the master socket? If so, the only migration option is to SOGEA (the digital-only version of FTTC) with digital voice. It sounds like you know that already.
Zen's digital voice will happily migrate your existing PSTN number, and yes, you'd plug your handset into the voice port on the router they supply. Alternatively, you could choose to migrate your phone number to a third-party VOIP service to make it completely independent of your broadband (which is not as simple as the all-in-one Zen offering, but it does give you flexibility for easier broadband changes in future)
(*) Check at broadbandchecker.btwholesale.com and click on the "Address" tab. If it says "WBC FTTP", then you have it. If it says "FTTP on demand" then you *don't* have FTTP available.