Problems with T-mobile

Due to an Illness my husband lost his job in June of 2008, we had to make payment arrangements with T-mobile so we did not lose our service with them. They did not honor their agreements, before our first payment was due they sent our account to the collections and when we called them to tell them that our first payment wasn't due yet why did they send it to the collection, they replied they do not have information about us that we would have to talk to the collection agency. They will not honor their agreement and now they want $600.00 more then what we owe. Can you please help with this? We have the agreement letter from them and we have the collections letter that shows, before our first payment was due they turned over to the collection agency. All we want is to pay our bill so it is not on our credit, and due to a mistake by T-mobile we can't stop that from happening.