Asterisk IP Autho

I am searching for a long time now, but couldnt find. When I fix a extension to get connected to asterisk with IP authentification,
asterisk sends to the client registration error 403. Nevertheless, when the client makes a call,asterisk accepts the call and makes the call.
What I need is: Asterisk shall not send registration error, instead I want the client to be registered to asterisk.
The combination I am using is asterisk 1.6.x, freePBX 2.9.x, A2billing 1.9.3

Best Regards