SIP trunks

Trunk types

The Vodia PBX uses SIP trunks to connect PBX users with the public telephone network. Most of the SIP trunks use SIP registrations to connect to the trunk providers, but the PBX also supports gateway trunks that don't require a registration. The gateway trunks are usually used with PSTN gateways that are in the office LAN, in the same network as the PBX. Both methods can be mixed, there is no limit on how many different SIP trunks a Vodia PBX system may use in an installation.

Dial plans are used to route outbound calls from the PBX to a trunk. As with the number of trunks, there is no limit on the number of dial plans that a system can have. They can be assigned to extensions and to other accounts that may trigger outbound calls on the system.

Global trunks are useful in multi-tenant environments when the domains share a single trunk instance provides by the hosted PBX provider.

Inter-office trunks are used in companies that have offices in different countries. Those trunks make it possible to terminate calls using the PBX in the country where the call will be routed to. This reduces cost for international costs.

Predefined trunk providers

Vodia maintains a list of know SIP trunk providers. Those providers can usually be configured only by providing the username and the password. After setting up the trunk, the PBX will register to the right destination and set up the SIP headers the way the SIP provider wants it. There is no further tweaking of the SIP settings neccessary, which makes it easy to use SIP trunks.

PSTN gateway setup

Similar to the automatic provisioning of VoIP phones, the PBX can also automatically set up PSTN gateways with a reasonable configuration. For this the administrator needs to provide the IP address of the device (typically a statically assigned IP address) and make sure that the device is using it's defauls username and password. After the initial configuration, the PBX will set up a dial plan for the new trunk. The administrator can use this plan as a template for more elaborate rules for dialling out.

CO-line emulation

Central Office-lines were used to subdivide physical trunk lines into single call appearances, especially when using T1-lines. This feature is useful to manage the trunk resources. The functionality can be emulated using the Vodia PBX CO-lines.

CO lines are "seized" when placing outbound calls or receiving inbound calls. This is useful when users likes to emulate PBX using shared lines from the old days when lines were analog. Vodia CO-lines can be limited to handle only inbound or outbound calls. This makes it possible to keep a certain number of inbound or outbound calls available in busy offices.


Trunks may have rates that define how much a call to a specfic destination will cost. Those rates depend on the starting pattern of the destination. Inbound calls can also be billed depending on the number that was dialled, making it possible to show the cost for toll-free numbers. VoIP phones that support advice of charge (AoC) can show the cost on the display while the call is progressing.

The cost for the calls can be fed into a monthly bill using a cloud-based billing platform. Rates can then include semi-flat rates, where users get a budget of monthly included minutes.


Because trunk are essential resources in the PBX and Internet connection can be challenging, the SIP can send emails when the regsitration status changes on a SIP registration trunk. This is very useful when troubleshooting random behavior with SIP termination.

As with the extensions, SIP trunks can also trigger the recording of the calls that are handled by the SIP trunk. This feature is very useful to troubleshoot problems with SIP trunk providers and is welcome material to the support staff.

Caller-ID presentation

Especially in environments when US and international numbers are mixed, it is useful to tell each trunk on how to interpret and present numbers. The PBX will put the numbers into a global format that abtracts from the local represenation.