Unresponsive Store Manager

I got my kids cell phone for Christmas this past year. While I was making the purchase, the store manager told me that if I import a number, I'll get $400.00 off per device. He told me I had 30 days to import the new phone number, and the credit would appear on my account. I was disappointed that I couldn't take advantage of that deal because these were my kids first cell phones, so we didn't have an existing number to import. He told me I could still take advantage of the deal; all I had to do was create google voice phone numbers and import those. So that's exactly what I did, even requiring the assistance of another employee at the store to make sure everything was done correctly. When my first bill with the additional devices on came, there was no credit. I was assured the credit would appear, I just had to give it some more time. Next bill...no credit. When I went into the store to get the issue resolved, I was given the managers cell number and told to call or text him. I did that, and was again assured that he had submitted a "ticket", and was following the situation and would make sure the credit was added to my account. Next bill comes....no credit. At this point I reach out to Xfinity directly and am told that there is no record of a ticket being submitted on my behalf, and furthermore, that that promotion never even existed. I take this information back to the store manager and I'm told AGAIN that he is handling it. It has now been 6 months of billing and this is still not resolved. I'm not interested in talking to the store manager any longer because he clearly is giving me the run around. I'm hoping that posting here will give me some insight as to what I can do to rectify this.