Getting over charged by T-Mobile... and not getting any help!

I have the same problem as every one else. Customer Service is a joke. My local store even told them they were overcharging me. Then I canceled my service and now  I am stuck with a $500 bill for my phone and I had to cancel my service for being over charged and dealing with rude Customer Service when I tried to explain to them that they overcharged me. I even offered to give the phone back which they wouldn't take. This happened several times on my bill and I had to keep calling them and every time they were rude to me! I even recorded the phone conversations because it got so bad! Do not get service with them!