You wouldn’t concern yourself with the legality, if your threat vector includes the traceability of the phone number.

And regarding your (in this context) nonsensical privilege remark: I live in such a country. Yet I have used such numbers.

If somebody is that paranoid (or in a situation where that level of secrecy is necessary) they would not use a number that is traceable to them… So it doesn’t matter if they have your phone number or not.