My Pearl would do this sometimes while I was on YouTube...I just figured it was draining the memory too low and forced the device to reset. Never had it happen during a call...sorry I can't help anymore than that.
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Had this exact thing happen for the first time ever. Using 4.5.0.93. I was having a problem with random reboots on 4.5.0.81 so I changed the OS. Everything seems fine except for that one incident yesterday while I was on a call. About 20 minutes into the call the line went dead and when I looked at my 8120 it was rebooting. I was not near a land line so it took about 5 minutes before I could call back. Nice. I hope this doesn't happen again or it's back to .69 for me.