If the copper internet connection at the site at present is fibre to the cab(fttc), you can simply change to a sogea service same thing but no normal dialing tone.this is a data only line, and the phone number for the site at present, can be used on a digital phone plugged into the modem.so no need for any copper connection to the telephone exchange.so it is an alternative to full fibre,and 4g/5g.
The call centre is probably set up for sales so have a vested interest to sell full fibre.
But as your post stated , if its low use and only used for bookings and admin stuff,4g/5g modem and have the land line diverted to a nominated mobile number, cheap and cheerfull.The deadline is a while away anyway(and will come and go as per)