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Knowledge document #1637

Setting up an ISDN telephone system for an ISDN BRI in the FRITZ!Box

Point-to-multipoint ISDN lines are common in home environments and in small businesses, while ISDN BRIs are used in professional environments. The "FON S0" socket on the FRITZ!Box is designed for ISDN devices that are suitable for use on ISDN point-to-multipoint lines.

If you set up the FRITZ!Box as described here, you can also use an ISDN telephone system (PBX) suitable for use with ISDN BRIs on the "FON S0" socket. Then you can use all of the telephone numbers configured in the FRITZ!Box with the telephone system, for example all of the extension of a SIP trunk line that uses a trunk number and block of extensions (a SIP trunk line similar to an ISDN BRI).

Important:Since this is not a guaranteed product feature, AVM does not guarantee problem-free operation of an ISDN telephone system designed for use on an ISDN BRI.

1 Configuring telephone numbers in the FRITZ!Box

  1. Configure the internet telephone numbers in the FRITZ!Box.

2 Connecting the telephone system to the FRITZ!Box

  1. Connect the telephone system to the "FON S0" socket on the FRITZ!Box.

    Important:With the "point-to-point" operating mode, you can only connect one ISDN device to the "FON S0" socket.

3 Restarting the FRITZ!Box

When the FRITZ!Box is restarted, it recognizes that a telephone system designed for use on a point-to-point line is connected to its "FON S0" socket. After the restart, the FRITZ!Box automatically switches the "FON S0" socket to the "point-to-point" operating mode:

  1. Click "System" in the FRITZ!Box user interface.
  2. Click "Backup" in the "System" menu.
  3. Click on the "Restart" tab.
  4. Click the "Restart" button.

4 Configuring the telephone system in the FRITZ!Box

  1. Click "Telephony" in the FRITZ!Box user interface.
  2. Click "Telephony Devices" in the "Telephony" menu.
  3. Click the "Configure New Device" button.
  4. Enable the option "ISDN telephone system" ("ISDN PBX"), click "Next", and follow the wizard's instructions.

5 Configuring the extensions of your telephone system

  1. Enter internet telephone numbers in the extensions on your telephone system exactly as they are displayed under "Telephony > Telephone Numbers" in the FRITZ!Box user interface.
  2. You can also use all of the extension numbers of a SIP trunk line with the extensions of your telephone system, even if you did not enter the extension numbers in the FRITZ!Box (see section "Configuring a SIP trunk line").

6 Disabling the energy-saving mode in the FRITZ!Box

The FRITZ!Box is equipped with a number of energy-saving mechanisms. For example, communication between the internal S0 bus and ISDN devices is terminated when there are no active connections. Some ISDN devices have problems with this function. Therefore, disable this function in the FRITZ!Box if ISDN devices are having problems establishing connections:

  1. Click "Telephony" in the FRITZ!Box user interface.
  2. Click "Telephony Devices" in the "Telephony" menu.
  3. Click the (Edit) button next to the ISDN device you want to configure.
  4. Click on the "Properties" tab.
  5. Disable the option "enable energy-saving mode for internal S0 bus".
  6. Click "OK" to save the settings.