The mechanism involves specific error codes sent back to the switching center (MSC) when a barred condition is met. For example, if a user attempts to dial an international number while "Outgoing International Barring" is active, the network intercepts the setup message and returns a release cause, typically Cause Code 55: "Incoming calls barred within the CUG" or generic call barring rejection messages.
If a barring condition is triggered, the network sends a SIP response code indicating failure. Common responses include: call barring feature