No circuit/channel available — congestion on the call's route.
Where you see it
In ISUP/BICC REL messages, 2G/3G DTAP Disconnect, and SIP Reason headers (Q.850;cause=…).
What to check
Check capacity/congestion on the SS7 trunks involved in the route.
Check the cause location (user, private or public network) to see which side released the call.
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