Sorry, I should have been more specific. In Europe (or Germany at least) it’s required by law that you provide an imprint with contact information for every site you host, as well as a privacy policy that includes contact information of your GDPR officer if you collect any kind of personalized data. Since I’m a one-person company, that includes my personal phone number since I don’t have a business phone. Also chrome webstore for example requires a phone number if you host an extension on there.

Edit: Also this wasn’t about collecting phone numbers, but about providing one for your business if you host a publically accessible site