My 650 (Sprint service) stopped ringing a few days ago. Either it doesn't ring at all, or it rings, the screen come son, but I cannot unlock the phone and answer it. Happens regardless of signal strength (even at full strength)

I have seen a few other people complain about this here, but no suggestions and resolutions yet. Mostly it's people with 700ws, but since I have a 650 I decided to post a thread to this forum. Any suggestions would be helpful.

I did not do any software installs or make any changes to phone, not for at least two months. Problem seems to be spontaneous.

Any help appreciated.