GSM call flow from MS to roaming MS-

The MSC sends ISUP IAM to GMSC, GMSC analyses the dialled
digits and figure out which HLR to query and send "Send
Routing Information". HLR always knows the exact location of
mobile in the form of address of MSC/VLR.HLR sends "provide
roaming number" asking MSRN(Mobile Station Routing Number).
MSC-serving provides the MSRN in return message and HLR
forwards it to GMSC which inturn sends IAM to MSC-S.

  MS       MSC     GMSC             HLR               MSC-S
              1)IAM
            -------->
                     2)MAP Send Routing
                      Information
                      ------------->
                                   3) MAP Provide Roaming 
                                      number
                                      --------------->

                                    4) MAP provide roaming 
                                        number(MSRN)  
                                                           
                
                                      <----------------

                     5)MAP send routing 
                     information(MSRN)
                     <---------------
                      6)IAM(ISUP
                      --------------------------------->