Most people use one mobile phone number. (unless of course you have a company phone as well as a personal phone)
So..is the phone number itself subject to the privacy laws and or privacy statements of websites?
Usage would be when you add it to a website for 2FA for instance. Is the website entitled to use it as an identifier?
Because it is almost unique it could be used for tracking purposes. It is a unique identifier to you.
I am not talking about phone calls, but because it can be associated with your interactions on a website it can be used as data and collated (by Google or Facebook etc).
It can be cross referenced with other websites.