They do, this guy insists on using his phone to join Zoom calls

I believe there are ways to disable phone for Zoom calls ... but that needs buy-in (you can disable audio conferencing, and say it's for "security reasons").

Surely a manager directly approaching the individual and saying “don’t do that” is a better approach?

99.9% of these problems could be solved by a manager doing their job, but that's the problem.

Why not to connect an external USB mic to a desired smartphone?