Hello. I tested some sdks and ready-to-use softphones and finally I chose Ozeki Voip SIP SDK because of the good support and fine voice quality.
Unfortunately it is a bit expensive but due to professional license you can also develop an autodialer system and IVR and thus create a whole call center with good sound quality for your wholesale business.
Check out one of its sample program on its homepage, e.g.,
How to implement Auto Answer?
Hopefully I helped you with these information.