i want to formally dispute $805.05 November bill

Hello I want to dispute the $805.05 billing i received for the month of November. Can someone tell me what are the procedures to formally dispute the large November monthly bill.

My name is Ezana Aimero (304412****) with a family plan of 3 lines. (Account number *********) i have been with T mobile/sprint for the last 15 years. Now i have been asked to pay 805.08 cents for international calls that i didn't use. I call Tmobile and Notified October 4th, November 9, November 13, and November 18th and both times i spent more than 3 hours and i spoke with supervisor and Manager and both people promised me to resolve the issue and get back to me with in 24hrs However no resolution provided. After 15 years with Tmobile, i really got frustrated. I am very sad and It received really poor customer service. Now i don't have no option so i requested T-mobile to terminate and transfer/port my number to a new cell phone carrier