(Code: **331* ) The roaming protection. You are on vacation. You have had three glasses of wine. You do not need to call a London hotel from Paris. This prevents expensive international dialing while still allowing local calls.
When you activate barring, your phone sends a USSD command (Unstructured Supplementary Service Data) to your carrier's switch. The carrier then updates your service profile in their Home Location Register (HLR). what is a call barring
(Code: **35* ) The witness protection mode. Nobody can reach you. Not spam, not your boss, not your ex. This is more aggressive than Airplane Mode because the network knows you are unreachable (callers get a fast busy signal). (Code: **331* ) The roaming protection
Apple and Android hide these settings deep inside Settings > Phone > Call Barring . You usually need your carrier's PIN (often "0000" or "1234") to access them. Manufacturers want you to use their sleek block lists, not carrier-level codes. You do not need to call a London hotel from Paris