Aha. That's a very different kettle of fish.
It needs something that will present as a telephone interface. Simplest to make is probably a telephone recording circuit like these.
http://www.epanorama.net/documents/t...elerecord.html
and to make it always off-hook with a 600 ohm resistor in parallel (or one slightly more so the combined impedance across the line is 600 phm).
You might also be able to adapt an old answering machine that answers the line, plays a 'ding dong' as the OGM, then you can speak. You'd have to wipe the tape/memory occasionally as it filled up.
The official version would be an interface like this
https://www.baldwinboxall.co.uk/bt-interfaces/
They do pop up on ebay varying from cheap to most spendy. BT5 maybe £150 used/unsold or about £400-450 new.