Comcast / Xfinity review: Xfinity mobile service

I had a phone Sim that was locked to Xfinity. I called in to get it unlocked so I can change carriers. The very first agent confirmed with me that she was able to unlock it. No other info was provided. I then tried to switch carriers and the phone was still locked. I then called back and explained the issue and was told they would need to unlock the phone by activating the mobile service to unlock the phone. They confirmed the phone would be activated and unlocked in 24hrs and that it will be canceled as well, and no payment needed. No other info was provided. I was still looking for a new carrier when I get an email stating a charge from Xfinity Mobile. I then call back and explain the situation to the agent. They inform me that the charge was because the plan was activated and never deactivate. I told them what I was told previously, and the agent told me that they made an error. That they would refund me the charged amount and make sure this time it was canceled. I asked the agent if there was anything else about the account I needed to know, and he assured me everything was taken care of. I then get another email a month later stating an even larger bill from Xfinity Mobile. I called in explaining the whole situation and was told there was nothing they could do. They stated that because there was some used data after they unlocked the phone that they weren't able to refund me the amount. I expressed that I was told the service wasn't going to be active after that initial call and that I stated clearly to each agent I never wanted the service and was going to use another carrier from the very beginning. I was never told of this last amount charged was on the account either. I asked if there was anything else on the account I needed to know and this very last agent assured me there was nothing else, but I am very concerned that I will need to address this whole situation again. Please help me.