Trunks in FreePBX 2.5

4 min read

A trunk in the simplest of terms is a pathway into or out of a telephone system. A trunk connects a PBX to outside resources, such as PSTN telephone lines, or additional PBX systems to perform inter-system transfers. Trunks can be physical, such as a PRI or PSTN line, or they can be virtual by routing calls to another endpoint using Internet Protocol (IP) links.

Trunk types

FreePBX allows the creation of six different types of trunks as follows:

  1. Zap
  2. IAX2
  3. SIP
  4. ENUM
  5. DUNDi
  6. Custom

Zap, IAX2, and SIP trunks utilize the technologies of their namesake. These trunks have the same highlights and pitfalls that extensions and devices using the same technology do. Zap trunks require physical hardware cards for incoming lines to plug into. SIP trunks are the most widely adopted and compatible, but have difficulties traversing firewalls. IAX2 trunks are able to traverse most firewalls easily, but are limited to adoption mainly on Asterisk-based systems.

In terms of VoIP, ENUM(E.164 NUmber Mapping) is a method for unifying E.164 (the international telecommunication numbering plan) with VoIP routing. The ENUM system can be considered very similar to the way that the Internet DNS system works. In the DNS system, when a domain name is looked up an IP address is returned. The IP address allows a PC to traverse the Internet and find the server that belongs to that IP address. The ENUM system provides VoIP routes back when queried for a phone number. The route that is returned is usually a SIP or IAX2 route.

An ENUM trunk allows FreePBX to send the dialed phone number to the publice164.orgENUM server. If the called party has listed their phone number in the directory, a VoIP route will be returned and the call will be connected using that route. A VoIP route contains the VoIP protocol, the server name or IP address, the port, and the extension to use in order to contact the dialed phone number. For example, a SIP route for dialing the number 555-555-1234 might appear as SIP:[email protected]:5060. This is advantageous in several ways. It is important to note that indirect routes to another telephony system are often costly. Calling a PSTN telephone number typically requires that call to route through a third-party provider’s phone lines and switching equipment (a service they will happily charge for). If a number is listed in the ENUM directory, the returned route will bridge the call directly to the called party (or their provider), bypassing the cost of routing through a third party.

ENUM also benefits the called party, allowing them to redirect inbound calls to wherever they would like. Service disruptions that would otherwise render a particular phone number useless can be bypassed by directing the phone number to a different VoIP route in the ENUM system.

More information on ENUM can be found at the following web sites:

DUNDi (Distributed Universal Number Discovery) is a routing protocol technology similar to ENUM. In order to query another Asterisk system using DUNDi, that system must be “peered” with your own Asterisk system. Peering requires generating and exchanging key files with the other peer.

DUNDi is a decentralized way of accomplishing ENUM-style lookups. By peering with one system you are effectively peering with any other system that your peer is connected to. If system A peers with system B, and system B peers with system C, then system C will be able to see the routes provided by system A. In peer-topeer fashion, system B will simply pass the request along to system A, even though system C has no direct connection to system A.

DUNDi is not limited to E.164 numbering schemes like ENUM and it allows a PBX to advertise individual extensions, or route patterns, instead of whole phone numbers. Therefore, it is a good candidate for distributed office setups, where a central PBX can be peered with several satellite PBX systems. The extensions on each system will be able to call one another directly without having to statically set up routes on each individual PBX.

More information on DUNDi can be found at the following web sites:

Custom trunks work in the same fashion as custom extensions do. Any valid Asterisk Dial command can be used as a custom trunk by FreePBX. Custom trunks typically use additional VoIP protocols such as H.323 and MGCP.

Setting up a new trunk

Setting up a trunk in FreePBX is very similar to setting up an extension. All of the trunks share eight common setup fields, followed by fields that are specific to the technology that trunk will be using.

In order to begin setting up a trunk, click on Trunks in the left side navigation menu as shown in the following screenshot:

FreePBX 2.5 Powerful Telephony Solutions

From the Add a Trunk screen, click on the name of the technology that the trunk will be using (for example, if a SIP trunk will be used, click on Add SIP Trunk) as shown in the following screenshot:

FreePBX 2.5 Powerful Telephony Solutions


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