H323Plus
Development framework for H.323 applications like softphones or gateways
H323Plus is the continuation of the OpenH323 project and aims to maintain full source code compatibility while adding the latest H.323 features.
License: MPL 
Website: https://www.h323plus.org
Support: https://www.willamowius.com/h323plus-support.html
API documentation: https://www.h323plus.org/api/annotated.html
Tutorial: https://toncar.cz/openh323/tut/