why does my phone just do one short ring instead of constant ringing?

Your line has what called a rectified loop fault on it.

A number of different things could be causing it, but more likely damp somewhere. Maybe a socket, a phone, or a damp connection somewhere.

BT will charge a small fortune for an engineer to c all out if the fault is found to be on your own equipment, so best to check at the main NTE ( socket).
There should be 2 small screws that you can undo, take the front plate off and test with a known working phone in the test socket.
If your line still has the funny ring on it in the test socket, ring BT as the fault lies with there wiring somewhere and no charges should be raised.

If the line rings OK, then the fault is on one of your extensions or pieces of equipment.

Good luck!

Andy - BT Engineer
December 2009