Is this an analog POTS line? If so, test set at frame will verify that it is good going into system. CNTRL C A screen is line test. Enter her Line ID - the result should be Y Y Y. X's are bad. If still doesn't work, move to another trunk port.
If it is NOT an analog POTS line, then none of the above is valid. Here's where your friendly neighborhood phone person comes in handy...
Mike