If if was a trunk issue I would be getting a lot of calls. 500 extensions... 3 T1s....
I had another user Friday with the same problem. I replace the handset, swapped the 8410D phone (used onc from a user close by, problem persisted), changed the port, the x-connect wire from the MDF to the IDF, from the IDF to the station cable. Now I've exhausted everything. At the end of my handset cord I changed her from a 1 pr digital port to a 2 pr digital port. No better. Her voice seems loud enough in person. Only two things left are: to watch her use the phone, and, put someone else on the phone. The problem might be like Mark C said "between the chair and the handset".

Candor - Intelligence - Good Will