Motorola V400

My wife's phone had constant call drops and I took it back to the vendor. He thought it was a phone defect since there was only one reception bar showing on face plate. He gave me a replacement phone and sent the original one back to the carrier. I now got a $50 bill from the carrier due to damage beyond economic repair. When I looked into this they told me there was "liquid damage--which could been caused by many sources including perspiration. Well, this phone was less than 3 months old and was never abused. I am protesting the $50 payment (the insurance I have for the phone excludes this liquid damage issue). Has anyone ever successfully protested a bill to a cell phone company?