I had a similar problem with my insurance.
Some things to check -
For my ins. co., you have to be claiming ED as a direct result of prostate removal or some other covered illness, and best that you not have any previous ED complaints, lest they say it was not directly caused by the surgery. Had to get a letter to that effect from my uro/surgeon for the pump and Trimix purchases. The prescription was not enough. My policy does not cover ED as a result of getting older.
Be sure you are claiming the pump as rehab / durable medical equipment (it will be best if it has a stated warranty, as they expect that it should last 2 years or more).
At each call where someone tells you something is covered, politely and unobtrusively ask for a written letter (not an email) to that effect. ("Hey, I'm really confused about all this, could you send me a confirmation?") Having the letter where I was told it was covered saved me.
I've also seen that if you go to one of the specialty providers that works in-network with your insurance, they will be a little better versed in how to get the claims through the system. That was the case with my Trimix. Denied to me when I asked, but the pharmacy knew the ropes, and got it approved.