[SR-Users] [Kamailio]: INVITE Authorization

Hello. I have Kamailio ( K in further ) and 2x Asterisk boxes ( A1 and A2 in further ) configured, so UAC registers at K and when it sends a call, it's routed to A1 or A2, balanced. The problem is, that I cannot find how to authorize INVITE requests, so unregistered UAC could not send INVITE requests. Simply cannot find anything. I'm making registration, using www_authorize() and checking all INVITES with proxy_authorize(). Just after kamailio is started - everything works fine and as planned: registered UAC can call and not registered - cannot. But after aproximately 40 seconds everything is stopped. Not calls passed and everybody receives 407 Proxy Authorization is required. So, the question: how it is correctly to verify that incoming INVITE on K is authorized? It seems to me that I'm doing that in wrong way. Thank you.