OCCAS always response with a 481 to a PRACK

Hi all, it my first this in this community.
I've developed a SIP Servel and deployed it on OCCAS.
I need to handle PRACK but if always see that OCCAS response with 481.
Can anyone help me?

INVITE sip:PSI_Conf@192.168.6.81:5060 SIP/2.0
Via: SIP/2.0/UDP 192.168.49.100:5060;origin=192.168.49.100
Route: <sip:webGTK@192.168.6.81:5060;transport=UDP;call=orig;lr>, <sip:dproxy.321321@192.168.49.100:5070;orig;lr>
Record-Route: <sip:dproxy.123012301230@192.168.49.100:5060;transport=UDP;lr>
From: CLI_A <sip:CLI_A@192.168.49.100:5060>;tag=1
To: <sip:PSI_Conf@192.168.6.81:5060>
Supported:100rel
Call-ID: 1-13910@192.168.49.100
CSeq: 1 INVITE
Contact: <sip:CLI_A@192.168.49.100:5060;transport=udp>
Max-Forwards: 68
Allow: INVITE, ACK, PRACK, CANCEL, BYE, OPTIONS, MESSAGE, NOTIFY, UPDATE,REGISTER, INFO, REFER, SUBSCRIBE