A company that resells AT&T Wireless/Cingular TDMA IS-136 service "Beyond Wireless" will activate any qualifying phone free with the requirement that you make a call at least every 60 days. Theoretically with their service you could have phone service for months on end and not pay anything unless you used up all your minutes. The drawback (you knew there had to be one) is that they only have local numbers in a limited number of locations so if the number wasn't local to people calling you they would pay a toll charge to call you.
Outgoing calls cost 15 cents/minute down to 10 cents/minute if you purchase a large number of minutes refill voucher. I have two former AT&T Wireless TDMA phones that I've activated on their service to use if the need arises. I should also add that calls that are not "on network" are charged at four times the regular rate so if you are in an area where AT&T Wireless never had service it would not be a good fit for you as any minutes you would use would be off network and would cost you four times the regular amount. If however you are in an AT&T network area it's a good deal especially for the casual caller.
Theoretically you could go to a resale/thrift shop and get a used AT&T wireless handset and have Beyond activate it for you and you'd have an extremely economical package.