[edited] Indeed it is far from straightforward that merely doing a video call suffices to check the keys. Signal is famously using a special protocol for secu

[edited] Indeed it is far from straightforward that merely doing a video call suffices to check the keys. Signal is famously using a special protocol for secure key sharing through the server, which I have not studied. But as said by another comment, there is no way around verifying explicitly the public key using an independent channel.