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. 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.
Sample program: voip-sip-sdk.com/p_354-implement-auto-answer-voip.html
Have a good testing!