This is a common complaint with the 7105T. I rolled out (20) of them and have taken back (2) because of this complaint.
Some things to think about:
1) The mic is right under the #5 key - so make sure when you are chatting on the phone you keep your face/cheek from leaning on this key.
2) Try your phone with a Bluetooth or wired headset to see if the problem still exists - if it does, then you may have a bad mic on your phone.
I doubt very highly its a software problem...its usually no problem at when once you learn not to lean on the phone, or its a hardware issue and then upgrade to the 8700.
My suggestion is to keep calling T-Mobile complaining and asking for help with this issue - regardless if they help you or not. Make sure after each call they give you a TROUBLE TICKET # - that way you know there is a record of it.
I hope this helps!
BES 4.0 SP4 HotFix#2 for Exchange 2003