Strange service-related question

I was over at a friend's place for a few hours last night. Historically, I never got very good service in his basement, and last night was no exception. However, whenever I'd check my phone, the service provider would occasionally say "1" instead of the usual "AT&T" or "No Service." I've sever see this "1" service before. Was I roaming on some "1" service or something strange like that?

 

T-Mobile G1 unlocked with an AT&T 3G SIM.

 

It was just the character "1" where the service provider normally is. Typically, I'd see an "AT&T" where I saw the "1".