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Knowledge document #73

Telephone connected to FRITZ!Box is dead and/or telephone system indicates a malfunction

A telephone connected to the FRITZ!Box is dead and/or a telephone system (PBX) indicates a malfunction:

  • When you pick up the handset of a telephone connected to an analog extension or the "FON S0" socket on the FRITZ!Box, you do not hear anything (for example no dial tone or busy signal). The telephone is "dead".
  • The display of an ISDN telephone system connected to the "FON S0" socket indicates that there is a malfunction.

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

Simply proceed as described below. After each measure, check whether the problem is solved.

1 Plugging the FRITZ!Box in

  1. Plug the FRITZ!Box into a power outlet where other devices work.

2 Installing the latest FRITZ!OS for the FRITZ!Box

  1. Install the latest FRITZ!OS on the FRITZ!Box.

3 Testing a different analog telephone

  1. Disconnect the telephone from the FRITZ!Box.
  2. Connect a different telephone to the same socket on the FRITZ!Box.
  3. If you hear a sound, then the first telephone is defective or the telephone cable is damaged or incorrectly wired. Test a different cable or contact the manufacturer of the telephone.
    • If you do not hear a sound, check whether the plug of the telephone is correctly wired. The two inner pins carry the analog signal of the RJ11 sockets of "FON 1" and "FON 2".

4 Setting up ISDN devices

Properly connecting ISDN telephones and other ISDN devices

  1. Always connect ISDN devices to the "FON S0" socket of the FRITZ!Box.
  2. Do not use ISDN cables that are longer than 10 m.
  3. Do not connect more than one ISDN device that must be supplied with power from the FRITZ!Box. You can connect a total of eight ISDN devices to the FRITZ!Box.
  4. Only use ISDN devices that are suitable for use on a point-to-multipoint line.

    Note:Point-to-multipoint ISDN lines are common in the private sector and in small businesses, while ISDN BRIs are used in professional environments.

Connecting ISDN telephones and other ISDN devices individually

The following steps are only necessary if two or more devices are connected to the "FON S0" socket:

  1. Remove the ISDN distributor and all ISDN devices including their cables from the FRITZ!Box.
  2. Test each ISDN device individually and attach it directly to the "FON S0" socket (without an ISDN distributor).
  3. If all ISDN devices function properly when no other ISDN devices are attached, test a different ISDN distributor.

Testing a different ISDN telephone

The following steps are necessary if only one device is connected to the "FON S0" socket:

  1. Connect a different ISDN telephone (for example from neighbors, friends) to the "FON S0" socket on the FRITZ!Box.
  2. Check whether the telephone makes a sound (for example a dial tone, busy signal) when you pick up the handset.
  3. If you hear a sound, then the first telephone is defective or the telephone cable is damaged. Test a different cable or contact the manufacturer of the telephone.

5 Restarting the FRITZ!Box

  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.
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