You could try
- AgoraRTC Extension by kevinkun, realtime communication with voice and/or video (10 USD)
- Video / Voice Calling Extension by Amit_Narwal (15 USD)
taken from the extensions directory App Inventor Extensions | Pura Vida Apps
EDIT: it looks like both extensions do not work anymore? It might help to contact the developers @Kevinkun @Amit_Narwal
Taifun