Debt made by someone else

by Ron
(Springdale, AR, USA)

My sister in law was living with this guy, he had a phone installed in her house in his name. When they broke up, he left owing a phone bill. He took the phone and my sister in law just ignored the bills that came in his name. In 2003, she moved into the house with my wife, her sister and me. For some time she has been getting calls and bills from Southwesten Bell for the amount of the bill. The colletor told her that the debt was her's because the phone was in her house. Is this true, or are they trying to make her pay because they cannot find him?