Map of AT&T/Cingular Network Cell Sites

Phones activated through Cingular prior to Summer 2005 prefer the T-Mobile network, but will switch to the Cingular network if the signal is weak.
Former AT&T Wireless GSM customers and phones activated through Cingular beginning late Summer 2005 prefer the Cingular network cell sites,
 but will switch to the T-Mobile network if the signal is weak.

Eventually all Cingular customer's phones will be switched over to preferring the Cingular/former AT&T Wireless network cell sites.