php & VoIP

Hello, I wrote an application for VoIP where the IP addresses are retrieved through some PHP scripts as well as all the mysql db updates; it works fine until the clients are connected to internet with a public IP address. As soon as a call is made to a client that is behind a NAT the connection fails because the real IP address (Private one) is not available. To reach this IP I need to send an UDP message to a IP address. I think I should use the sockets, but I have no knowledge about it. there is anyone that can help me? may be some manual or documentation or script? Thanks in advance Alessandro