Knowing someone's phone number very obviously doesn't grant you direct access to autojoin their calls with people. At _best_ you can attempt to tell if they're

Knowing someone's phone number very obviously doesn't grant you direct access to autojoin their calls with people. At _best_ you can attempt to tell if they're currently in a call or not by calling them, but even that is far from reliable (e.g. in places where voicemail isn't as much of a thing do not disturb is sometimes implemented by pretending to be busy or switched off)