Google oftentimes doesn't.

One of my accounts requires a phone number to verify every single time I login. I have no clue why, and it accepts absolutely any phone number.

I have no clue what the purpose is aside from forcing me to give sensitive information to other people when my phone isn't available or I'm traveling (which I've been forced to do already).

You'd think they would make it more clear ("verify your 2fa number"). Maybe they want to bait criminals into entering their own or something.