If apt update refuses to update #Jitsi's package list due to an unknown signing key:
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://download.jitsi.org stable/ InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B4D2D216F1FD7806
W: Failed to fetch https://download.jitsi.org/stable/InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B4D2D216F1FD7806
W: Some index files failed to download. They have been ignored, or old ones used instead.
then follow the instructions at https://jitsi.org/downloads/ubuntu-debian-installations-instructions/