AT&T Cellular was not the same company as AT&T Long Distance. They only used the name AT&T for branding purposes.
That being said, this question would be much better answered on howardforums.com They'll be able to tell you who shares what towers in your area. It is true that carriers do share networks in some areas, depending on if they use the same technology (Cingular & T-Mobile are both GSM providers so could theoretically share, Verizon & Alltel are both CDMA and share, etc...)
But almost all cell carriers have like a 7 day return policy, so you can sign up, take your phone home, and if you don't get a good signal in your house, take it back and void the contract. No harm no foul.