Sometimes when I have to answer a call from an unknown number, I’ll pick up but won’t say anything. A human will say something when they hear it stop ringing, but most bots listen for voice activity before they start their recording so they’ll just stay silent as long as you do.
Is there anything in the rules about where the online cancellation has to be housed? I can totally see shady businesses hiding it in a part of the site that’s not even on their sitemap and noindexed just to keep people from finding it.