Disputing a Bill

Im very unplease with t-moible 12/28/2011 at 6:32 i gave them a call regarding my phone, cause it was disconnect. The call went I the customer wanted to let them know that i was going to pay my bill at a letter time, i did put some money on it prior to the call. I ask them if my 2yrs. contract was up with them and she agree, but i told them i want to continue service with them and i drop my other line as well. With tht she inform me if i wanted to stay with them after i pay my bill my monthly bill will be 59.00, i agree. Know i you can see i was sent to collection very disappointed why was i sent to collection we made agreement. please help