Billing question

My bosses son had a verizon telephone which was damaged and decided that he wanted to replace it with an iphone, as we had an iphone I simply forwarded the calls from the verizon number to the iphone (ATT Network) so that he could still get the calls without having to distribute a new telephone number. I couldnt actually simply transfer the number for other reasons that are immaterial to the question.
For several months the call charges on the verizon invoice were minimal as the son is in school and seldom uses the iphone and the verizon unit was given back at an official verizon store so there was no way of making calls other than the forwarded calls to the iphone unit which was automatic.
After six months the monthly verizon bill suddenly went over $700 each month on autopay meaning that its impossible to see the calls as the way to access them is online and as they password is sent to phone, that they have, he is unable to retrieve it and despite promises this has yet to arrive in the mail.
As the guy nominated to get to the bottom of the mess I called Verizon which is where my question comes from, I was told by two levels of technical support that when using the ATT iPhone that the calls were showing up on the Verizon Invoice, which I say is not possible as a) they would also have to be on the iPhone bill which they are not and b) the unit itself is totally divorced from the verizon network in any way and calls are simply made from the ATT network.
Now I am in a position where technical support claims thats the cause i.e. its ATT or Apples problem even though calls were being made when I physically had the iPhone powered off on my desk where it has been for a few days while I try and sort this mess out.
Anyone ever heard of something as bizarre as this and what is my next step after a round of 32 calls, countless fruitless hours on the phone, around $2200 in charges from Verizon, 14 days and no progress not even a callback from Verizon.

 

I totally get that incoming calls to the original number would show on both invoices as it uses airtime and capacity on both networks but these are outgoing calls on the ATT phone that Verizon says would be charged on their network as well.
Having worked for a cellphone provider (in another country) many years ago I just cannot see how a network that is basically a completely different technology could even see those calls as the signals are not compatible but I had to listen while the IT support and their second and third level also came on to tell me that I was wrong and that indeed the ATT calls would also be billed by Verizon but when I asked how those calls didnt show up on the ATT invoice none of them could answer and gave the standard answer that its not their network so they couldnt comment.
I am still not sure how to get these amounts refunded

 

As expected the outbound calls from the iPhone say that they are from the ATT network.

Obviously as somebody that spent a lot of years working for a telephone company I am not surprised at the level of support from the telesales experts as thats the initial reason that I left that industry but how do I work through getting a rebate ?

Any ideas ?