A neat way to test phone lines on a Cisco voice gateway is to use a hidden command, CSIM.
This will simulate a call that will follow your dial plan and translation rules, the called number should ring (if dialed correctly) and any debug command (e.g. debug isdn q931) will give you a proper output.
Syntax is csim start xxx (where xxx= number to dial)
Example use is:
Router# csim strat 9003923455600
Keep in mind that the called party won't hear anything if he answers the phone and the the output would always display failed.
CSIM it's not documented and I will update this post to reflect any new info.
Also check this if using SSH
