Contacting Xfinity to file a demand letter

I'm disputing a bill for a defective phone sent to me via xfinity from a blatant lie from their support teaam about the phone being free. Then after 3 weeks besides all of the other problems the buttons o the phone fall off, and was told by support I had to pay to have it fix citing their 2 week warranty. Why was Motos 1 yr warranty changed to 2 weeks? I believe it was because they knew phones were defective. I'm requesting my acct be credited for what I paid towards the defective phone and the attempts to get me to pay for defective phone and service cease and be erased. I intend to pursue this farther if this can not be rectified.