3 blackberrys, 4 new SIM cards, and they all keep muting themselves!

I have had a blackberry for several years now... I am on my 4th with T-Mobile and have kept the same telephone number. A few years back I started having issues where my phone suddenly went mute so the person on the other line couldn't hear me. Sometimes the audio came back after a few seconds, but usually the other person would just think they lost me and hang up...

The screen never displays the "mute" icon when this happens, so it is not an issue of me pressing UNMUTE to resume speaking...

I called T-Mobile about this issue several times, and everytime I reboot, change the SIM card, change the phone, change the software, remove applications... and I STILL have this issue with my new Bold 9780 and new SIM card (probably my 4th or 5th since this problem started)! I can't figure out how this could still be possible! Is there something strange connected with my actual telephone number which I have had for the last 8 years? I have a family plan and no one else has this issue with their blackberries!

I have this issues with callers from both landlines and cell phones (including people in my T-Mobile family plan), I have had it in NYC and elsewhere in the country, and both with and without headset devices connected.

Any help would be appreciated as T-Mobile has been useless in helping me!