cingular cell phone

dallas, tx

my son had a contract with cingular - their service was horrible - either rude or would make promises not kept - bill was never the same - he spent hours on the phone trying to get things fixed - the store fronts would tell him one thing - customer service would say they had not authority - finally a phone rep told him to cancel his service if he was so unhappy - so he did. now he is being harrassed by a collection agency for $386.00. thus far he has not returned their phone calls. the lady is rude when she leaves messages. this is uncharted waters for us. any suggestions?