Video calls indeed almost always use UDP. TCP retransmission isn't really useful since by the time a retransmitted packet arrives it's too old to display. Worse, a single lost packet will block a TCP stream. Sometimes TCP is the only way to get through a firewall, but the experience is bad if there's any packet loss at all.

VC systems do constantly send back packet loss statistics and adjust the video quality to avoid saturating a link. Any buffering in routers along the way will add delay, so you want to keep the bitrate low enough to keep buffers empty.