A virtual phone number can be given out instead of your personal number. Reasons for keeping your personal phone number private vary, but all reasons are valid. Your phone number is often closely linked to your identity -- especially if you've been stable enough financially to have had the same phone number for many years.
If you have a personal, non-PSI Google account as well as a valid US phone number you can sign up for Google Voice (here's the link for set up instructions).
Don't want to use Google? There's a few paid services out there. The features and pricing varies, so I've outlined the most important information.
one additional line; seems geared more for professional use
one additional line that can be easily changed; seems geared for additional privacy that would be needed for selling things online or dating; prepaid option available
multiple additional lines that can be easily changed; seems geared for additional privacy that would be needed for selling things online or dating; prepaid option available