Most likely you could lower the price of those calls if they are made via a A-Z service company instead of a company like vonage.
A small gateway, like a Quintum A200,or a Cisco ATA 186, and a account with a provider A-Z most likely will ower the prices to 50% of wath you are paying to Vonage.
We are talking about maybe a couple of hundred dollars on hardware an an account that you could open with $50 dollars worth of prepayment.
If you could provide the destinations where those calls are going to, I could tell you what is the going price for comercial service to those locations.
I could provide assistance with a small setup like that for free if you need it.
Just post a emai address where you could be reached.