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Knowledge document #135

Using an ISDN telephone system with the FRITZ!Box

If you connect your ISDN telephone system (PBX) to the FRITZ!Box, the devices connected to the telephone system can use all of the telephone numbers entered in the FRITZ!Box.

Requirements / Restrictions

  • The ISDN telephone system must be suitable for use on an ISDN point-to-multipoint line. If the telephone system is designed for use on an ISDN BRI, proceed as described in this guide.

Note:All instructions on configuration and settings given in this guide refer to the latest FRITZ!OS for the FRITZ!Box.

1 Configuring telephone numbers in the FRITZ!Box

Configure all of the available telephone numbers in the FRITZ!Box:

Configuring internet telephone numbers

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

Configuring landline numbers

  1. Configure the landline number 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.

3 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.

4 Configuring the extensions of your telephone system

If you do not assign telephone numbers to the extensions of the telephone system, the main telephone number configured in the FRITZ!Box is used to make outgoing calls. In addition, all of the extensions react to all incoming calls. Therefore, assign the desired telephone numbers to the extensions:

  1. In the telephone system, define which numbers the extensions should react to when you receive calls and which number they should use to make calls. Enter the numbers in the telephone system as they are displayed in the settings for the ISDN telephone system in the FRITZ!Box user interface (see fig.). If you assign an internet telephone number to an extension, and that number is identical to a landline number, outgoing calls from this extension will be made over the internet.

    Note:Sometimes a number may be too long for you to enter the complete number in the ISDN device. If this is the case, do not enter the entire number. Instead, enter as many of the last digits as possible.

    Fig.: Settings for the ISDN telephone system in the FRITZ!Box user interface

5 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 "Apply" to save the settings.
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