This guide will show how to configure an AVM Fritzbox router / firewall as a border device with 3CX Phone System in the LAN. The tests were made using a FritzBox 7170 with firmware version 29.04.76

.IMPORTANT:  The FritzBox has an integrated SiP Server inside. This makes the usage and natting of ports 5060 TCP and UDP to the 3CX Phone System impossible because the Fritzbox will not let you. Hence we have to configure 3CX to listen for SiP on a different port.
Note: This document will show screenshots in German as we could not upload an English language firmware on the device. However some settings are translated from German to English.
Related articles: Configuring AVM FritzBox as a PSTN Gateway (PUT LINK)

Part1) FritzBox Setup

Step1: Activate expert mode on the router

• Press on “Einstellunden” (Settings) in the top bar.
• Click “Erweiterte Einstellungen” (Advanced Settings) /  “System” (System) / “Ansicht” (View)
• Activate here the “Expertenansicht aktivieren” (activate expert view) box.

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Step2: Enable NAT for SIP requests through the AVM FritzBox.
This option is only available and needed when you use the Fritzbox in “LAN Mode” and not for usage in DSL Mode.

• Open “Erweiterte Einstellungen” (Advanced settings), “Telefonie” (Telefony) and then the “Erweiterten Einstellungen” (Advanced settings).
• Check the box for “Portweiterleitung des Internet-Router für Internettelefonie aktiv halten” (enable port forwarding of the Internet-Router for internet calls).
• Set the value to 5 minutes.

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Step3: Set the NAT rules to the PBX.

• Open “Erweiterte Einstellungen” (Advanced settings), “Internet” (Internet), “Freigaben” (Sharing) and pick the tab “Portfreigaben” (port sharing).
• The following example shows the pbx on the IP: 10.172.0.141
• The port for SIP CAN NOT BE 5060 as normal and must be changed.
• The Example shows it for port 5062

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Step4: Add the ports you need in the FritzBox

• SIP-UDP: 5062
• RDP-UDP: 9000-9049
• Tunnel-TCP: 5090
• Tunnel-UDP:5090

Part2) 3CX Phone System Setup

Step1: Setting up the 3CX PBX

Due to the Fritz.Box would fatch all traffic to 5060 we have to change the SIP Port in the 3CX Phone System.
In the fritz.box port forwarding we have already set the SIP Port to 5062 for TCP and UDP.
Use this port as well in the PBX.

Access the 3CX Management Console, click on Settings / Network and change the SIP Port accordingly to 5062

Important: Global changes that need to be made in this scenario

A) The provisioning files will be updated but the phones will need to be re-provisioned with the new settings.

B) All phones in your network have to register to Port 5062 and not more to 5060 as usual. An example is shown below:

Example: Configuration of a Snom Phone

Example: Configuration of a 3CXPhone

Related articles: Configuring AVM FritzBox as a PSTN Gateway (PUT LINK)