Wirelessly posted (8700g: BlackBerry8800/4.2.1 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/102)
I have that same problem withmy TMo BlackBerry as well in the Birmingham area. If the TMo signal goes dead, the BlackBerry will not grab the present Cingular signal.
I have inquired numerous times with TMo about this and have been told over and over again that is because Cingular will not give TMo roaming agreements in our areas.
I am pretty certain you will find that to be the case in your situation as well.