Get over the paranoia.
First, VoIP is unlikely to work well thru a 3rd party proxy or VPN because of increased and variable latency. To fix the problem you will need to use your real IP address anyway.
Second, SIP providers are frequently defrauded or used for nefarious purposes by people in certain countries. The providers don't like dealing with that, and it raises costs for their honest customers, so they take reasonable steps to protect themselves. Those steps often include extra validation or delays for customers from those problem areas and blocking access from those who are obviously attempting to hide their internet address.
Third, it becomes a customer satisfaction and support nightmare when people are trying to run VoIP thru a 3rd party proxy or VPN, see the first point. It's better for the provider to be clear up front and simply prevent attempting to work thru known problem paths rather than have customers tie up support lines and post bad reviews because of poor call quality caused by their own paranoia.
Finally, there are some providers who will work with you if you have a legitimate reason. Contact their support but expect to justify yourself and jump thru some additional hoops. I'm not going to recommend anyone but you can look at
broadbandreports.com for more information.
