there isn't one. but that's irrelevant because the services that require your phone number don't do it for the sake of security or to combat bots, they do it to gather data. when you give a megacorp your phone number, they get your real name, your relationships, and a myriad data points to sell.

this is the reason why all megacorp mobile apps and every other free flashlight/calculator/alarm app require access to your contact list.