I imagine they say something like, "I couldn't see or here you. Did you see me?" through the other channel, and because modern tech messes up enough, it'll be believable to a lot of people. Especially the most gullible.
HN gets so caught up in logic. Has no one watched interviews of scammed people? Huge numbers of them report that they were suspicious it was a scam but they were so worried for a loved one they sent the money anyway because it was worth the risk to them just in case. Anything that helps push scams in the even slightly plailusible direction will make scams more successful.