The referenced equipment (IMEI) isn't recognized (unknownEquipment).
Where you see it
In a returnError component of a MAP dialogue over TCAP/SCCP (roaming, SMS, call routing).
What to check
Confirm the IMEI is correctly provisioned/known where expected (e.g. in an EIR).
Check which network element sent the returnError (calling GT) and the operation it answered.
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