Adtran 908e responds with 501 method not Implemented

Hi All,

I am sending Register messages from our MiVoice Office 250 to one of our service provider through Adtran 908e. What I observed is MiVoice Office 250 is constructing Register messages as required by the service provider but Adtran is responding with 501 method not implemented.

Please find the below SIP messages. What could be the possible reason ?

Request-Line: REGISTER sip:IPINET1.com:5060 SIP/2.0

    Message Header

        Via: SIP/2.0/UDP 192.168.10.162:5060;branch=z9hG4bK1448869845-968

        Route: <sip:192.168.10.236:5060;lr>,<sip:209.115.XXX.XXX:5060;lr>

        Max-Forwards: 70

        Allow: NOTIFY,REGISTER,REFER,SUBSCRIBE,INFO,INVITE,ACK,OPTIONS,CANCEL,BYE

        User-Agent: Mitel-5000-ICP-6.2.5.93

        From: <sip:p587XXX@IPINET1.com>;tag=Mitel-5000_1448870625-968

        To: <sip:p587XXX@IPINET1.com>

        Call-ID: 1448867239-968

        CSeq: 1 REGISTER

        Expires: 1800

        Contact: "p587XXX" <sip:p587XXX@192.168.10.162:5060>

        Content-Length: 0

Session Initiation Protocol (501)

    Status-Line: SIP/2.0 501 Not Implemented

    Message Header

        From: <sip:p587XXX@IPINET1.com>;tag=Mitel-5000_1448870625-968

        To: <sip:p587XXX@IPINET1.com>;tag=4fceacc8-7f000001-13c4-3dd-3649280-3dd

        Call-ID: 1448867239-968

        CSeq: 1 REGISTER

        Via: SIP/2.0/UDP 192.168.10.162:5060;branch=z9hG4bK1448869845-968

        Content-Length: 0