I have the EXACT same problem! And this is a replacement phone bc the last one which I only had for 2 months had stopped working due to the trackball not working and would start sending 15 blank e-mails at a time, muting and unmuting during calls, and a ton of other weird things.

I get a replacement phone yesterday from T-mo and I immediately start having probs with not being able to answer or make calls due to the phone shutting down after about 5 seconds into the calls. WTF!