How can I escalate for a billing issue that hasn't been fixed for half a year

I purchased a new iphone back in Dec with a promised $500 promotion if I port in a new line. But the new line was never ported in correctly even though I went to a local store multiple times trying to resolve this. The billing for the new device suppose to be around $30 per month but I have been charged $50 per month. I've spent more than 20 hours so far on the phone with more than a dozen Xfinity customer service agents. Each of them promised me the issue will be fixed. They have broken the promises every single time. I've worked with tier 1, tier 2 and tier 3 agents. An agent told me they have submitted escalation request to the upper management in May, but now another agent told me she didn't see any request submission in their system. She told me she would do this correctly (on 6/6) and promised to call me back by 6/16 with a solution. Never got a call. Now I am on the line again with multiple agents and starting all over again. 

How can I escalate this to someone who actually can resolve this?