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SIP line registrar concepts

In SIP, a registrar is a logical entity that stores information about where to contact a user when someone dials that user's number. In CIC, a registrar represents a SIP carrier. Before service is granted, the SIP carrier usually requires CIC to register the SIP line, which is done by TS sending a SIP REGISTER message to the carrier. The registration tells the SIP carrier that if it receives calls for CIC's DID (Direct Inward Dial)s, then those calls should be sent to CIC.