What is Source Identification? 

Source Identification (SI) of incoming calls is a critical part of the phone system, as attempts to match each incoming INVITE message to a configured SIP Trunk, or reject the call if matching fails. This is especially important on systems with multiple SIP Trunk accounts configured, from the same or different providers.

What Does Source Identification Do? 

The purpose is for 3CX to correctly identify which SIP Trunk each incoming INVITE message must be associated with. Multiple methods can be employed, however as best practice, all SIP Trunks are recommended to ensure that one of the following SI matching methods is used:

• rinstance parameter match.
• Unique Call Source Identification criteria match

The rinstance parameter, whenever a SIP Trunk Provider supports it, ensures that the system immediately knows which SIP Trunk to associate an incoming call with, and because this value is unique per SIP Trunk, it allows users to add multiple instances of the same SIP Trunk providers in 3CX without causing an issue. The limitation of this however is that it requires the SIP Trunk provider to be registration-based, as this is not available for IP-based providers.

For IP-based SIP Trunk providers, or for Registration-based providers that don’t return the rinstance parameter, the second best option is to make use of the Call Source Identification options, that provide the flexibility to choose any SIP field and compare it either with a static value found in the SIP Trunk, e.g. the Registrar FQDN or IP, or a static value that can be entered manually (Custom Field).

Using the Call Source Identification options may however result in disabling multiple SIP Trunks of the same provider to be configured in the system. This follows a logical approach, depending on the information an INVITE message contains from a provider.

What Does This Mean for My Calls?

This means that when the phone system does the decision-making is for each incoming INVITE, at every colored shape in the below. flowchart, the phone system iterates through every configured SIP Trunk in random order and checks if the specific Source Identifying matching criteria is met. If not, then it will move onto the next colored shape repeating the same process, stopping at the first match it finds.