Theoretically speaking, what ivlok said is the tweak you're looking for if your service provider is doing packet inspection to filter out voip. A vpn would encrypt packets and your actual endpoint will not be your phone. The service provider will see a large amount of data but wont understand what it is and so should have no reason to filter it.

However, the latency of the traffic will increase considerably. Voice depends on low latencies - otherwise both parties notice a lag - almost like an international call, but without the distance.