I think the best way to do this is to use a passphrase so that it's clear that it's not just gibberish but you have the benefit that it's random text. Obviously at the end of the day, it all comes down to the person on the other end of the phone but I suspect they'd be more suspicious of someone saying "it's a bunch of gibberish" when they can see "grumpily siberian pampers panorama unroll aloof masculine mandatory" versus "YpZVpyQHsmPATt1P" (also the former is much easier to read over the phone).