Where to file a formal complaint

I searched the entire website trying to find where to file a formal complaint, but since there seems to be no real place, it is being left as a question.

Xfinity is one of the worst service providers I have ever worked with. Every time I call, I get transferred a million times and after an hour+ of sitting on the phone (multiple times over the past few months), they say that they have resolved my issues and then I receive a bill again for things they say they have removed.

I am being charged for a phone I returned over 2 months ago and have been going back and forth with Xfinity for months about. They told me multiple times that the phone was received with no issue and that the charges were being removed, but then I called again today (due to being charged AGAIN), and they said the phone is still spending review and is being rated as a "C." I only had the phone for a few days, and I barely used it due to transferring back to my old phone a few days later. I couldn't even find a way to remove my card on my account and had to ask the person on the phone.

The site is not helpful. The phone system [Edited: "Language"]. The staff need to be trained better. 

I am working to cancel all my services with Xfinity and will never work with them again. [Edited: Inflammatory]