MSRN Mobile Subscriber Roaming Number is a temporary number which is used during the call setup procedure, during a MTC call it is assigned to the subscriber by the VLR in which the subscriber is roaming/visiting and is passed to the GMSC or the MSC from where the call is originating via the HLR.
MSRN is passed to HLR once it requests the routing information from the VLR in which the MS is roaming

The originating MSC than routes its call to the MSRN to complete the call, MSRN expires once the call setup procedure completes and again becomes available to be used for another call. Every MSC is given a pool of MSRNs