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Knowledge document #1139
FRITZ!Box 7390
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Crackling noise when conducting landline calls on an ISDN line

When conducting landline calls on an ISDN line, you always hear an annoying crackling sound, regardless of whether the telephone is connected to the FRITZ!Box or directly to the ISDN NT.

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

1 Checking the cabling

Faults in the cabling often go unnoticed. Therefore, make sure that the FRITZ!Box is correctly connected to your line:

  1. Check the cabling to your landline.

2 Connecting only the FRITZ!Box to the ISDN NT

The following steps are only necessary if other ISDN devices are connected to the ISDN NT in addition to the FRITZ!Box:

  1. Disconnect all ISDN distributors, all ISDN devices including their cables, and all other cables from the ISDN NT.
  2. Connect the FRITZ!Box to the ISDN NT using its original cable. Do not connect any other devices.
  3. If the FRITZ!Box still does not function correctly, continue with the next section.
    • If the FRITZ!Box functions correctly when no other devices are connected, the error is not caused by the FRITZ!Box. Faulty cables, a defective ISDN device, or an incorrectly terminated S0 bus or distributor may be the cause. Normally, the following measures can eliminate the cause of the disturbance:
      1. Do not connect ISDN devices using cables that exceed 10 m in length.
      2. If the problem occurs when using an ISDN distributor: Connect a preassembled resistor to a port on the ISDN distributor, unless the ISDN distributor already has one or the ISDN distributor is not terminated.
      3. Connect all ISDN devices directly to the ISDN distributor using the shortest possible ISDN connection cables. Do not connect any further S0 bus extensions to the ISDN distributor.
      4. If an S0 bus was installed: Remove all branches from the S0 bus that do not directly lead to an ISDN device.
      5. Make sure that two 100 Ohm resistors are used to terminate the end of the S0 bus and that termination is turned on in the ISDN NT. If you have an S0 bus installation with the ISDN NTBA in the middle of the bus, termination resistors must be switched on both bus ends. In this case, you must switch off the resistors in the ISDN NTBA.
      6. Remove all ISDN cables from the ISDN-NT and/or S0 bus that do not have ISDN devices connected to them.
      7. Do not use more than 12 ISDN jacks. Do not connect more than eight ISDN devices to each ISDN jack.

3 Replacing the DSL splitter and ISDN NT

  1. Replace the DSL splitter and the ISDN NT. Try to avoid using the same device types.

    Note:You can obtain a new DSL splitter and ISDN NT from your DSL provider.

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