The baresip and pjsip are two well established open source projects offering SIP/VoIP libraries and client applications, many SIP softphone implementations use them. One can found Python bindings for both of them:

• https://github.com/OpenJarbas/baresipy
• https://docs.pjsip.org/en/latest/pjsua2/building.html#building-python-swig-module