How does 3CX Phone System decide whether a call is internal or external?

When is a call internal or external?

Calls are decided to be either local or external based on whether the contact IP of an inbound SIP packet is local or not. To check whether the IP is local, 3CX Phone System uses the following process:

  • It queries all IPs assignedto the PC on which it is installed.
  • Using the subnet masks assigned, it checks whether the IP is local ACCORDING to windows.
  • In addition, it also checks a configuration file for a list of local subnets. This configuration file can be found in the <inifilename.ini>.
  • Local subnets are listed under the section <sectionname> in the following format <format>.

If the IP is considered local, then the call is internal. If the IP is not local, then the call is external. Additional Information on Transcoding:

In Transcoding mode, RTP can be encoded with different codecs by each SIP entity. For example, RTP from SIP entity A to media server can be encoded with GSM and RTP from SIP entity B to media server can be encoded PCMU. Media server will transcodes these two codecs. In our example RTP packet being sent from SIP entity A to SIP entity B, are received from the media server in GSM codec, media server decodes them and encodes them in PCMU and then they are sent to SIP entity B.

If transcoding takes place, one does not need to configure codec priority. The proper negotiation will be done automatically by Media Server.