[sneak]

It is completely anonymous, there is no link from the telephone number used to register to the person using it.

[lxgr]

Yes, but you only have one number/identifier that links you across all contexts. That’s a pseudonym, which is a different thing from being anonymous.

You’d need multiple phones and a phone number per context to be actually anonymous. That’s just not practicable for many people, I’d say.