VOIP service with an API that can be resold?

Question

Looking to add dialing capabilities to my desktop application. Requires:

1. API/SDK to allow seamless integration into C#. An application user would have no idea who the VOIP provider is.
2. Allows re-sale in license. We include it as part of the package and charge our end users

Any packages out there that fit this bill?

Answer 1

If you're looking to wrap up full softphone capabilities the main ones I'm aware are NCH's Express Talk and pjsip. I've never attempted to integrate with either so don't know how easy or hard it will be with C#. If you only need your application to be able to intiate and/or control calls you could use any SIP stack, an open source C# one being sipsorcery.