I am located in Knoxville, Tennessee. I seriously doubt the phone switches to packet data from GSM, at least I haven't noticed this. The only thing I have noticed is that it works much longer (7-8 days) with some SMS message centers and only 1-2 days with some others. I have been dealing with this for about 2 months now since I got the phone and I have pretty much tried everything (from not enabling bluetooth to removing applications, upgrading firmware etc.).
At this point the only reason I can think of is that I used to be an AT&T customer and having an old SIM card, AT&T branded, somehow the phone is not compatible with it. But if this is the case how can it even work to begin with, even if it is for a few days.
The phone must be stuck in some kind of different mode though since it consumes battery at 2-4x the rate when it stops receiving SMS messages. And it is beyond me why sometimes I have to reformat it 2-3 times before I start receiving again.
Cingular has been of no help to me either but they are the only GSM choice here.
BTW, I really don't send any SMS messages, I just receive messages from work via SMS.
What I am trying now is +12099042010 as the SMS center, "Character Encoding" to "Full Support" from "Reduced Support" and "Message Validity" set to "Maximum time".
If anybody has additional ideas please let us know.