I once signed up for an account and didn't give a phone number. Everything was fine and I filled out my profile. Later that day my account was restricted for "suspicious activity" ( I hadn't done anything except fill in the bio ) and the only way to reinstate it was to verify my account with a phone number.

Now, I am not saying it was a nefarious way to get my phone number. It's more likely a side effect of processes designed over time by many people.

But if I were trying to sneakily require someone to give me a phone number without explicitly saying so, that is how I would do it.