FRITZ!Box 6590 Cable Service - Knowledge Base

FRITZ!Box 6590 Cable Service

Using an ISDN PBX with the FRITZ!Box

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

Requirements / Restrictions

  • The ISDN telephone system must be suitable for use on an ISDN point-to-multipoint line.

1 Configuring telephone numbers in the FRITZ!Box

First, configure all of your telephone numbers in the FRITZ!Box:

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

2 Connecting the PBX to the FRITZ!Box

  1. Connect the PBX to the "FON S0" port on the FRITZ!Box.

3 Configuring the PBX 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 PBX", click "Next", and follow the wizard's instructions.

4 Configuring the extensions of your PBX

If you do not assign telephone numbers to the extensions of your PBX, then the main telephone number you 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 PBX, define which numbers the extensions should react to when you receive calls and which number should be used to make calls. Enter the numbers in the PBX the way they are displayed in the settings for the ISDN PBX in the FRITZ!Box user interface (see fig.).

    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 PBX 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 device is cleared when there is no active connection. Some ISDN devices have problems with this function. Therefore, disable this function in the FRITZ!Box if ISDN devices are having problems with establishing connections:


  1. Configure the telephone so you can send keypad sequences (sequences of symbols and numbers, such as *121#) and enter special characters such as * and # with it. On many telephones, you can configure this function in the menu "Operation at a PBX > Dialing options > Dial * and #".

Disabling the energy-saving mode

  1. Enter #966*1* on your telephone and press the call key (lift the receiver).
  2. Hang up again after about 3 seconds.

    Note:You can reenable this function by entering #966*0*.