No route to the destination at all -- a routing/configuration issue, not the called party being unreachable in the usual sense.
Where you see it
In ISUP/BICC REL messages, 2G/3G DTAP Disconnect, and SIP Reason headers (Q.850;cause=…).
What to check
Check the routing tables on the originating/transit side for this destination.
Check the cause location (user, private or public network) to see which side released the call.
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