How much exactly do you need to protect a phone number?

I’m talking about privacy and security reasons, not spam issues (although spam can have phishing).

How much exactly do you need to protect a phone number?

I don’t think there is a universal answer that is more specific then “As much as you care to protect it.” It depends on you, your goals, and your threat model. I personally consider it to be a rather personal piece of information, so I try pretty hard to limit who knows my personal number.

A phone number is typically an identifier that is unique to you that most people rarely or never rotate or change, it can and is used to link you across accounts, and to identify you (or verify you as you). It is also a pretty hard identifier to perfectly protect (since your real world friends, family, and colleagues most likely expose their contacts list to Google/Apple/Meta/etc and possibly other unsavory companies. For these reasons individuals have good reason to try to limit the sharing of their personal number, and data hungry corporations have good reason to try to collect your phone number along with other datapoints.

I guess the two most relevant questions to ask/answer are:

  1. How important is protecting your phone number for your goals and threat model?
  2. To what degree is it actually practical or possible to protect your personal number?