It means that the “called party” is already IMSI DETACHed. Basically, it means the MS didn’t send a LOCATION UPDATE message to the network, or that the network failed to decode it. Therefore the network assumes the MS is detached.
Check that the timer controlling the periodic location update is correctly set (is it the timer T1202 ?? i’m not sure), try to reduce it to 3 hrs or 4 hrs.
There is also a timer at BSC or NSS side, telling how long the NSS should wait without getting a LU message from a MS before deciding it is detached.. check this timer (timers ?) as well. Check 3GPP, it’s all described in there.