I work with Grandstream VoIP phones, and most of their phones support OpenVPN.  I've built a simple OpenVPN server to support them.  It is located on the same subnet as the PBX and routes traffic to/from these phones without affecting the local network on either the PBX or the phone end of the connection.  It works great but all my users want to use it for softphones, but there is no softphone, to my knowledge, that has OpenVPN (or any VPN) support built in.

Do any of your softphones support OpenVPN?

Is it something you would be willing to develop?  I think such softphones could be sold successfully for aroung US$50 each.  

How much would it cost to have you develop such a smartphone version?  I can't pay much but I think I could sell enough of them to pay for some development.

Both Android and iOS would need to be supported.  The simplest way to understand what is needed is to get one of my server boxes and a Grandstream phone.  The key is that the softphone's communication goes over the OpenVPN connection while the rest of the smartphone's traffic goes over whatever connection it is configured to use.

Please let me know if you have any questions or suggestions.  Thank you.