I have not had any lock ups with my phone, but this past Friday it did stop receiving text messages. I was still able to send text messages. I called tech support and the first thing they had me check was the available memory (Settings>SD card & phone storage).
Tech said any time the available memory gets down to 15% or less of the phone memory this can happen. Can't remember exactly what he said the total available memory was, something like 128 MB. My phone wasn't even close to 15%.
Next, he suggested I try deleting some text threads as they can get corrupt (?). I deleted a few and later all. Didn't may any difference. He made a suggestion that I try resetting something on the phone, but before I did that, I rebooted the phone and it started receiving texts again. Perhaps that was it, or it was a combo of deleting text message threads and rebooting.