If your phone line has a constant engaged tone, or no dial tone at all, then there may be a fault somewhere on the line or in the exchange.
Firstly you need to check your internal equipment. Try the phone line with another handset just to rule out that your handset isn't broken.
If you also check that all of your internal wiring and hardware is not faulty by plugging your phone directly into the master sockets test socket. On the master BT socket you can detach the faceplate with two screws, under here is a phone connector. If your phone works when you plug your handset into the test socket, there is a problem with some of your internal wiring i.e phone cable, phone, micro filter, cable extensions.
If your phone still does not have a dial tone or is fully engaged when plugged into the test socket, this is a fault only a BT engineer can fix.
Please report this to our support team at firstname.lastname@example.org or 03332025000.