The network was unable to page the UE (Unable to page UE) -- it couldn't be reached over the radio.
Where you see it
In the Cause IE of a GTPv2-C response (Create Session, Modify Bearer, Delete Session…) on S11, S5 or S8.
What to check
Check radio coverage/registration state for the UE at the time of the attempt.
Check which node sent the response (MME, SGW or PGW): the cause belongs to that node's decision.
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