Marty Fremen expressed precisely :
Correctlt, in the UK

If this is what you have at the moment, you must also have a single
line which is carrying your ADSL.

Eh? this should be the case with your existing two-line analogue number
and it would certainly be possible and normal with ISN2e.

That's BT for you. Rentals are dearer, generally & the PBXs are a bit
dearer too.

PS. 3 line, 8 extension Analogue Panasonic PBX circa £300