Dial *67 before the phone number you want to call
On iPhone, go to Settings > Phone > Show My Caller ID, then turn it off
On Android, open the Phone app settings and disable Caller ID or set it to hide number
Contact your carrier to request permanent caller ID blocking
Use a calling app or VoIP service that lets you hide your number
Check whether your country or carrier supports caller ID blocking codes
Note that some emergency and toll-free numbers may still see your number