All disputes, even if given to somebody over the telephone have to be made in writing or they can be considered never to be made. The federal laws very clearly state this, and it should be so indicated on the back of your bill with the terms something like "you may make a claim over the telephone, but your rights will only be preseved if followed up with a letter".

It's an important lesson everybody has to know, to preserve your rights you have to follow up with a letter.