This blog post will explain how to route calls to a PBX. Whenever an incoming call comes in through a SIP provider, they need to know where to send it to. Two methods of routing calls is SIP registration and host based routing. SIP registration allows a phone system to be authenticated by a user name and password. That phone system sends a REGISTER request to the provider periodically. That means the provider will be informed of the phone systems IP address and port number. This allows a provider to route calls to a phone system. Advantages of using SIP registration are the ability of a phone system to be assigned a dynamic IP address, several devices can use the same SIP trunk, an SIP trunks status can be monitored, and more security because a user name and password will have to be sent to the provider. Host based routing allows a phone system to be authenticated by a static IP address and port number. That means a phone system does not have to send a REGISTER request to the provider. No credentials are sent over the Internet when an incoming calls is routed to a phone system. Host based routing is useful if a phone system has a secure IP address. Not much configuration is needed for a phone system to use host based routing. Advantages of using host based routing are being able to reduce bandwidth usage for the phone system and provider, a phone system can easily be identified by its IP address and port number, and eliminates jitter while the call is being set up. Some features that come with an SIP trunk require a static IP.