Just my 2¢, for what it's worth...
CALL T-MOBILE TECHNICAL SUPPORT! Since you'd be trouble-shooting your BlackBerry, do NOT call from your T-Mobile BlackBerry. From a different phone line (your office phone, your home land line phone, if you have one, your significant other's cell phone) call either 1-800-937-8997 or 1-877-746-0909 and tell the automated menu system you want "BlackBerry Technical Support". Whenever I've had a really unusual problem with my BlackBerry, I've found them to be EXTREMELY knowledgeable and VERY helpful.
And this way, if there's a genuine problem not with your phone, but with the cell tower in your area, they'll know about it and can open a trouble ticket on the issue.