Once the communication server or the IVR system is administered to create UCIDs, these products assign a UCID to each call. For incoming calls over ISDN or SIP trunks, the communication server determines whether or not the call already has a UCID. If so, the communication server preserves the existing UCID and does not create a new one. If the call does not have a UCID, the communication server creates one when call processing begins.For incoming calls over trunks other than ISDN or SIP, the communication server does not create a UCID for the call because these trunks do not support the transmission of UCID.

For outgoing calls, the communication server creates a UCID when the caller goes off-hook.