It's most likely a coverage issue, where Cingular has good coverage there (either because they are under a tower or because they have a repeater installed) and the other companies don't.

Type of phone and position in the building could make a world of difference.

T-mobile (and probably others) have street level coverage maps that at least around my house are quite accurate

http://www.t-mobile.com/coverage/

Toss in the address and see what they say they have for coverage.