You are partially correct.
If you have acces to both ends of the VOIP circuit, it will be very easy.
Either set up VPN ,or use the router to change the ports and confuse the ISP provider.
There is a provider that will take calls from you in a non-standar port.
Maybe many other provider do that. I only asked to the guys in Dual Talk and they could provide the service.
By the way, I am not related to the company or get comission, only a informed user.
Wilson Boyrie.