Comedy idea. Not sure if it'd be legal.

The main reason it's annoying when someone is talking loudly on the
phone on a train is that the conversation is one-sided and your brain
can't ignore the oddity of it not making sense.

So, my idea is to record the conversation and then overdub it later
with a silly voice completing the conversation. The trick would be to
make it a credible but utterly ludicrous exchange.