I hope someone can help...

I have att/cingular w/ my 600 - it has been randomly dropping calls after only thirty seconds or I dial to send and then it goes back to the original screen, like I just turned it on (my wallpaper).

I upgraded to a locked 650 - slipped my SIM in and same issue - dropped calls and unable to even start calls. It's not a coverage issue - I'm calling from my office with no previous issues and four bars.

Anyone have any ideas?