Video calls alone won't stop a MITM attack. They would just send both video streams along, and record both sides.
Signal does have the capability to have a verification phrase displayed, which is generated from the session key. Reading that off can make the video more difficult to MITM, because then they'd have to morph the audio to match the phrase, and if it's done after the video is setup, morph the video as well. Not impossible, but difficult.
Signal does have the capability to have a verification phrase displayed, which is generated from the session key. Reading that off can make the video more difficult to MITM, because then they'd have to morph the audio to match the phrase, and if it's done after the video is setup, morph the video as well. Not impossible, but difficult.