As explained few posts above, I have connected my Grandstream GDS3710 (a SIP enabled video door bell), a tablet running Android (and webrtc), using asterisk in between based on the doordroid.js card made available by @rdehuyss (Thanks a lot!!). I am now able to see and talk to the person ringing on the video door bell and opening the door (if I want to).
I have shared my project here https://community.home-assistant.io/t/sip-doorbell-android-tablet-and-ha-integration/136171?u=greengolfer
GV