Regarding your last question, each state has a Statute of Limitations on how long a debt is legal. Go to the link below and look up your state.
The first thing you need to do is get those silly collection agents off your back. To do that, use the sample letter below requesting that they "validate" your debt. That means they must send you a copy of the bills, contracts, and anything that was used to calculate the debt you owe. If they don't supply you with that information, they can't keep the negative report on your credit history.
Send this letter to both Cingular and the collection agent. If they do not respond back in 30 days, send a copy of that letter, plus a letter demanding that the report be removed from your credit history, to each of the credit reporting agencies. That will get it off your records.
Now, what I'm going to bet is one of two things happened. They either messed up your last payment and didn't enter it correctly (you do have a copy of the canceled check, right), or you are being billed a fee for canceling your contract early. All of the cell phone companies have a very large termination fee, and there is no easy way to avoid it. It's right there in the contract you signed.....
....which is why you are sending them that "validation" letter. No contract = no legal agreement to pay them.
So make them prove your debt or leave you alone!