Hello! Free softphone solutions are good at the testing period but if you want to develop further, they lack many important requirements. So expensive but highly reliable sdks turn out to be cost-effective later and more applicable. I chose Ozeki VoIP SIP SDK and I have been using it with relatively few troubles in c sharp. The support team has always helped me in my job, and the sdk offers high compatibility with pbxes and well-known programming languages. It’s a bit expensive, but its quality and the good support compensate its costs. Any opinions about it are welcome.

Sample program: voip-sip-sdk.com/p_354-implement-auto-answer-voip.html