Moto v400 volume goes really low sometimes after switching to T-Mobile

I have a V400. It worked fine when I was with Cingular.
After I switched to T-Mobile. The phone's volume acted
strangely. I can make one call and hear the voice all fine.
The next call the volume goes almost non-existant, but I do
hear a very very small sound.

To fix the problem, I turn off and back on the phone. I
even open the backcover and remove the battery for a few
second. Sometimes it works. I can get to make a few good
calls, then all of sudden, the volume goes down to very low
again. And it only happen before a phone call. Never in
the middle of a phone call.

After a long talk with a T-Mobile rep, she said my phone is
bad. What a coincidence! I wish I didn't switch the
service. Can anyone offer some help?