GMM_IDENTITY_REQUEST decoding problem

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Hello,

I am decoding GMM_IDENTITY_REQUEST and it always shows GMM Cause with cause = Illegal MS .

raw data = Buffer 0000 - 000F 08 15 03

Decode output:

Message Content
    GSM A-I/F DTAP - Identity Request                                                                                                                             
           Protocol Discriminator: GPRS mobility management messages (8)  
            .... 1000 = Protocol discriminator: GPRS mobility management messages (0x08)  
            0000 .... = Skip Indicator: No indication of selected PLMN (0)  
        DTAP GPRS Mobility Management Message Type: Identity Request (0x15)  
        GMM Cause  
            GMM Cause: Illegal MS (3)