ISP blocks VOIP- Can Tunneling be used with VOIP Providers?

[sigertronic]

So I have managed to successfully make a remote call to my test IP PBX remotely, and did so using tunneling as a workaround for our country's sole ISP which actively blocks VOIP. Now I want to know if tunneling can be used in conjunction with a VOIP Provider to extend the value my customers get from 3cx beyond office bridging and into international calling.

Is it possible to use tunneling to route traffic between the IP PBX and a VOIP provider?

Beyond that, do any of the supported VOIP providers provide a means by which ISP limitations can be bypassed?

Thanks!