Dial *67 before the phone number to block Caller ID for that call
On iPhone, go to Settings > Phone > Show My Caller ID and turn it off
On Android, open the Phone app, go to Settings > Calling accounts or Supplementary services, then disable Caller ID
Contact your mobile carrier to request permanent Caller ID blocking
Use your carrier’s per-call blocking code if different from *67
Use a VoIP app or business line that supports anonymous calling
Check local laws and carrier rules before hiding your Caller ID