I have been having random problem with Nexus One since the first day I got it. (Stock Firmware, Stock Firmware-rooted, Cyanogen-beta3)
Symptom : Once in a while, when making a call you will hear just one ring on Nexus One, then silence for another two rings (the other end is still ringing), then the call is cut off. --- When dialing problem shows up, you won't be able to receive a call either. When the phone rings and you slide to receive a call, it will just hang up right a way.
Temporary Fix : I found temporary fix for this issue by restarting the phone, or just killing all the running apps.
This is my first android phone, so I'm really not sure if I done something wrong. I did some search but found no topics covering this issue.