MSRN is a temporarily telephone number assigned to a mobile station which roams into another numbering area. This number is stored in HLR and used by the Home Network forward incoming calls for the mobile station to the network it visits.
Why it is needed
On of the main feature of mobile network and specially mobility is to hide the location of the target mobile from the originating mobile. MSRN helps to achieve that, when source mobile calls the target mobile the HLR find that the mobile is not in the home network and forward the call to the new number and all is hidden from the source mobile hence protect the location.