[kanox]

As far as I can tell most new apps require a phone merely for registration. It's not even close to optional.

Twitter pretends it doesn't but it locks you out in ~5 minutes if you don't verify your phone number.

[zimpenfish]

But on the plus side, if you then create another account and use the same phone number, I assume that phone number is now linked to the new account (because they tell you that it's owned by another user and it'll be claimed) and not the one you actually want to use.