Double Billed and Want Refund

I opened an xfinity internet account and was told that I needed to mandatorily get a free mobile line. So, I signed up for that.

Now I find out that I have been billed twice for the past 11 months - Once for my internet +free mobile account and once more for a separate mobile account 

I have been talking to xfinity helpdesk for the past month and they opened fraud cases and investigations and said that yes I was double charged.

After multiple ECMs I was finally told that I would get the full refund in the amount of 823.08 within 5-7 business days.

That did not happen

So when I followed up through my 7th call, was told that the full refund would not happen and that I would only get 4 months of refund even though the double billing was done through no fault of my own

I asked to speak to a supervisor and he stated that even though I was double billed, I would not get  the full refund per policy.

He then offered a yes or no option (Yes to 4 months refund or No with no refund) and a 1 minute window to answer Yes or No. 

When I asked for more options for getting the full refund he starting counting down and then disconnected the call 

This is unacceptable. Why is there no exception process to get  full refund when I was double billed through no fault of my own.

How can I get my full refund?

Thank you