The "Trunks Module" is used to connect your FreePBX/Asterisk system to another VOIP system or VOIP device so that you can send calls out to and receive calls in from that system/device. You can create connections with Internet Telephone Service Providers ("ITSPs"), with other FreePBX/Asterisk systems, with commercial VOIP phone systems, with FXO Gateways (a device that connects an ordinary telephone line with a VOIP phone system using a network connection), and with FXO cards (cards that are installed in your computer and allow you to connect a standard telephone line).
If you don't have a Trunk set-up, you can still make calls, but only to other extensions on your same phone system.
The “Trunks Module” works together with two other modules that you need to know about: The “Outbound Routes Module” and the “Inbound Routes Module.”
When a phone on your system attempts to dial a phone call, the number it dials is compared against the rules set in the Outbound Routes Module. If it matches an allowed rule in the Outbound Routes Module, the call is then sent to the Trunk(s) specified in the Outbound Routes Module. Until you set-up your Outbound Routes, you won’t be able to make any calls, even if you have a trunk set-up.
For example, you can configure the system so that when a phone on your system attempts to call a local number or 911, the call will go out over a Trunk that is attached to a regular phone line. You could also set up your system so that long distance calls are routed over an Internet Telephone Service Provider. While you set-up the connection between your local phone line and your ITSP in the trunks module, you tell FreePBX which calls to send where in the Outbound Routes Module.
When a call comes into your system on a Trunk, the remote system will send your system the phone number that it is trying to reach and, sometimes, a Caller ID for the person calling. In the Inbound Routes Module, you will specify the phone numbers that can reach your system and what FreePBX should do with them. For example, you could configure your system to route calls to 212-555-0001 to extension 100, while routing calls to 212-555-0002 to extension 101. If you don’t have anything specified in the Inbound Routes Module, Asterisk will play a disconnect recording to the caller.
See the User Guide