I've both BT (analogue) and NTL (digital) lines in my home. The phones are interchangeable. There is NO difference in the devices you can plug into the respective sockets. I also have Broadband on both... and can say that Digital is by far, way superior, not only in performance but also in reliability in adverse conditions (your Digital DSL modem is NOT the same as your analogue ADSL modem).

There is only ONE exception. Those are the 100% 24/7 monitored alarms (ie they are permanently connected to a remote monitoring station) and because they are permanently connected, the remote station detects when someone has cut the phone lines to your house. You would need to speak to your monitoring company provider about the change.