Welcome to the community!
Does your phoneline connect into the back of your router or into the wall through a master/extension socket?
We’re moving home phone services over our fibre network instead of a traditional telephone line, this will be true for all landline services by 2025. You can find more information here, if needed.
If your landline has already been switched over to fibre then you may need to look at alternatives and device compatibility.
Fax machines are analogue devices and are not designed to work over IP networks. If your fax machine is very old it is unlikely to work, so you may need to reconfigure it or connect it to an adaptor of some sort that translates analogue signals to IP packets. You'll need to check with the manufacturer to check if it's compatible with our fibre network.