“A cheap router that could change the port as well as forwarding will do the trick also.
But you will need to have access to both ends.”
You mean that I sould use routers at both ends but thats impossible because any a-z providers do it only on 1720 or 5060 and they say that they are not able to change configuration.
I have come to conclusion that it can be done only with point to point private voip net.
Regards.