do you see that IMEI associated to that specific Samsung subscriber when you list registered subscriber in MSC/VLR? If yes,there is something internal problem in creation of CDR , if No , even subscriber send IMEI your Network cant understand it and put it as an Unknown Sub then I think if you are not allowed to Unknown IMEI to be updated they cant be even registered to Network.
I suggest you to force subscriber to perform location update (Delete it form VLR) then trace MS and try to find MAP_SERVICE_CNF and see what is it indicating–> unknown equipment, Illegal ME or it is White. At least at this point you know what happened between MS – MSC – EIR
another feasible matter , verify EIR GT whether it is defined properly or not( I know it is rare to happen 😉 )