Registration in openIMS with Asterisk as Application server

We are trying to integrate Asterisk PBX with the openIMS setup. The Asterisk in this case will behave as an AS. The problem that we are facing is as follows:

While registering with IMS, a Third Party registration is also to be done with the PBX. While doing so, after receiving the register message, PBX sends back a 401 unauthorised message to the S-CSCF. As per IMS standards, S-CSCF is not supposed to respond to any such message from any AS. It only expects a 200 ok.

Please let me know how the authentication procedure at Asterisk can be turned off.