MT Call procedure

Hi everyone. In MT Call flow must be as below    MS                                                BTS                   <- Setup               Call Confirmed ->         <- Assignment Command          Assignment Completed ->                   Alerting ->                   Connect-> but in implemented code in mnccms.c file immediately after sending Call Confirmed message, Alerting message is sent from MS to BTS , and this lead to ignorance of Assignment Command from BTS. WHY the procedure implemented as this ???