Insurance Back Pay

I see the same psychiatrist once a month and have for years and years now. I've been paying out of pocket, as my insurance didn't cover this psychiatrist when I first started seeing him. Last week, I found out they *are* covering this particular doctor, and have been for three years.

I called my insurance company (Select Health Community Care, Medicaid) and asked about it. I spoke with a very nice gentleman who told me he'd look into it. I asked about backpay (I'm on disability benefits and could desperately use the money to pay for other medical things), and he said he'd look into that, too.

I'm expecting a call back from this gentleman on Wednesday, and I wanted to be prepared. What can I do to ensure I get the most back pay possible? I live in Utah, in the United States. Thank you!