Is your phone number “private”

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.