Billing and phone trade in

At the beginning of January I traded in my

phone with Xfinity because they promised me a cheaper rate then AT&T in switching my service ($7 a month for my phone) In the store they had to submit a ticket because he did something incorrectly. So he submitted a ticket and kept my phone at the store. I noticed my bill never changed from $28 a month for the phone even after calling support 3 times. So I went back into the store they still had my phone, they instructed me to send it in and said that the problem was that never happened. So once they received my phone my bill would be updated. Well I went back into the store after my phone had been received and no billing change. They said that they cannot locate my phone and that it never should have been sent in. Since then the manager has said she is working on it but it has been over 2 months since they “recieced my phone” (I have the tracking info!) and no resolution or even close. I am at my wits end about what to do. I don’t understand what kind of company wouldn’t recognize that this is 100% their fault and update my bill for me. 

I have been on the phone 4 times for this issue, gone into the store 4 times and have had weeks of text messages with the manager.