Freepbx register with opensips

I’m currently trying to get my freepbx send call though opensips but I’m having issues when I attempt it to register and authentic with opensips. when opensips sends back a 401 unauthorized the freepbx doesn’t reply with username and password. I added register details in freepbx in the opensips trunk under register string putting in the following details "PBX:[email protected]" ( is the opensips server) but nothing happens when the authentication is switch off it can make a call though. Am I missing something else?

Sorry when I send an Invite from the freepbx it send back a 407 Proxy Authentication request and send an ACK back but dosn’t follow up with another Invite.