It entirely depends on your ISP of choice and for some ISP's it depends where in the country you are.
With BT you can keep your landline but the computer will decide during the order process whether the landline will be provided via the copper or via VOIP/Digital Voice.
The main difference being the phone is plugged in to the BT Hub on Digital Voice rather than plugged in to the BT phone socket on the wall.
I ordered BT for a neighbour around a month ago on FTTP and the computer automatically picked Digital Voice.
There was no way to change this to use the copper line despite 1 being present.
If your mother's exchange area is an area where BT have their Digital Voice ready and rolling out you may well find there is no way to sign up to FTTP while keeping the copper line active for calls.
If the area isn't yet using Digital Voice they can simply change your mother's service to Digital Voice at any point during the contract.
My mother joined BT FTTP around 2 years ago and retained her copper line for calls.
Around 9 months ago she was informed the line would be switching from copper to Digital Voice.
In the letter she was sent she was advised that if she had an alarm that required the copper line she could delay the switch to Digital Voice (I believe it said for a year) to give time to find a solution to the alarm. She took up that option.
However around a month ago she received a BT Smart Hub 2 in the post along with notification that the line was switching to Digital Voice.
This time there was no option to delay it and the switch happened last week.
The PSTN switch off is happening in 2025 and BT plan to be ready ahead of schedule.
They may not be the best choice of provider if your mother is insistent on having a copper phone line.
Edited by j0hn83 (Thu 16-Sep-21 11:24:58)