In Chapter 7, you saw how to terminate calls in a PSTN gateway. These days, most of time, you will terminate your calls in a VoIP Provider. To protect your gateways, you probably used a firewall preventing any other person from accessing the gateway's SIP channel directly. When you receive a call from the gateway, a trusted table is used to authorize the calls and your gateway is controlled inside your network. There are at least four ways to connect to a VoIP provider using OpenSER; let's see the pros and cons of the solutions:
Your VoIP provider authorizes your IP, and will bill according to the source IP. This is very common and I have seen several times. It is very simple, but it is definitely not the safest method. Some VoIP providers will require authentication.
If your VoIP provider requires user authentication, the standard way to do this is to use a B2BUA such as Asterisk. You configure Asterisk as an ordinary gateway, but instead of terminating calls in the PSTN it...