Using CM 6.1.2 I have a 7921 phone and 7945 phone configured to use the same static phone number.
The phones both ring when someone rings the associated extension number, trouble is when the call is answered on the 7945 phone the 7921 continues to ring.
You can resolve the issue for a few hours if you pull the battery out of the 7921 and reset it but then the fault reappears.
I have tried using a different 7921 handset but the problem happens again.
Am thinking it could be something wrong with the config of the phones but am at a loss as to what it can be...