[SR-Users] Non-authentication setup of Kamailio

Hi there, I have never used Kamailio before but want to investigate if if will work in my scenario before I invest time to learn it. I am the administrator of a small Wireless ISP. We do not provide SIP channels to our customers, we rather let them choose a service provider of their choice. Currently our customers are using two different providers. Since we are using ADSL with only one static IP we sometimes run into issues at the providers side with one way audio when our clients make a call to another client which is using the same service provider. I assume this is because of NAT. Since the RTP traffic actually leaves our network and then comes back to the other client the quality of the call is not as good as phoning a non client from the network. I want to know if it would be possible to setup Kamailio to keep the internal network calls traffic from leaving the network, and also allow free phone calls on our network. To do this we would require the authentication to take place on the service providers sip server and not on Kamailio. Kamailio would only do routing of the traffic and not worry about authentication. Would this be possible? Regards, Carel Burger