Cavalry Portfolio Services, LLC collect a bogus debt Phoenix, Arizona

As with appareantly many others, Cavalry Portfolio is trying to get me to pay a debt for cell phone use, occured after I was stationed in Korea.

Once I received my orders to report with my whole family to Korea, I cancelled all usual phone/cable/papers etc etc contracts. Believe me, when working for Uncle Sam in uniform you get real good at this.

After we get to Korea, we receive a letter from Sprint telling us we used our phone and owe x amount of money. So I stayed up until the wee hours to actually get to talk to a person in their "normal hours" collection department and straightened the issue out. No problem.
Not so fast, now, three and a half years later Cavalry comes riding along wanting a piece of the action.
So I find a lot about this in the rip off pages, but no real answer. What can be done to a company that is working like this???? Did anyone come to a conclusion with Cavalry yet? Does anyone know the best way to approach them that has some hope for success....other than ruined credit reports??

Peter
APO AP, California
U.S.A.