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The `F33f`_`[ISDN`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=ISDN]`_`f Services User Part (`F33f`_`[ISUP`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=ISDN_User_Part]`_`f) `!Circuit Identification Code`! (CIC) is part of the `F33f`_`[Signaling System #7`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Signaling_System_7]`_`f which is used to set up `F33f`_`[telephone calls`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Telephone_call]`_`f in `F33f`_`[Public Switched Telephone Networks`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=PSTN]`_`f`:cite-ref-1[`F5bf`_`[1`#cite-note-1]`_`f] as part of the Initial Address Message (IAM).

When a `F33f`_`[telephone`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Telephone]`_`f call is set up from one subscriber to another, many `F33f`_`[telephone exchanges`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Telephone_exchange]`_`f will be involved, possibly across international boundaries. To allow a call to be set up correctly, where ISUP is supported, a `F33f`_`[switch`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Telephone_exchange]`_`f will signal call-related information like `F33f`_`[called`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Called_party]`_`f or `F33f`_`[calling party`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Calling_party]`_`f number to the next `F33f`_`[switch`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Telephone_exchange]`_`f in the network using ISUP messages. The CIC provides information about where the voice part of the call is carried - on which trunk and in which timeslot.

>>References

`:cite-note-1`!1.`! `F0af`_`[↑`#cite-ref-1]`_`f ITU-T Q-Series Recommendations

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