This is exactly the problem with my phone, except mine oh-so-considerately waited until it was fifteen months old and thus out of warranty to start freezing up. The Cingular store in Tyson's was a whole lot of no help when I asked if there was anything that could be done to fix it, which doesn't surprise me since the first words out of the guy's mouth when I handed him my phone were "I've never seen one of these phones before". :-P Long story short(-ish): not eligible for a new phone at a discount from Cingular until the end of October (and they don't offer this model anymore, it's long-since discontinued), current phone can't be repaired under warranty, so I see eBay in my future and I don't know yet when my cell number will be in service again.
[Edit: Will try a different store tonight to see if it might be a problem with the sim card that swapping might help. If I'm lucky, that'll do it, but from what I've found it doesn't seem promising.]
[Son of edit: Even without a sim card in the phone at all, I get the same problem. It's apparently something screwed up in the phone itself. Snagged a
Motorola V635 quad-band phone on eBay.]