FRITZ!Box 7490 Service - Knowledge Base

FRITZ!Box 7490 Service
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Crackling noise when conducting landline calls on an ISDN line

You always hear an annoying crackling sound when conducting landline calls on an ISDN line. It does not matter 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 persists.

1 Checking the cabling

Incorrect cabling often goes 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 hubs, all ISDN devices including their cables, and any other cables from the ISDN NT.
  2. Use the original cable to connect the FRITZ!Box to the ISDN NT. Do not connect any additional devices.
  3. If you still experience the same problem with the FRITZ!Box, proceed with the following section.
    • If you no longer experience the problem when only the FRITZ!Box is connected, then it is not caused by the FRITZ!Box. The problem may be caused by a faulty cable, a defective ISDN device, or an incorrectly terminated S0 bus or hub. Usually, the following measures can eliminate the cause of the disturbance:
      1. Do not connect ISDN devices using cables that exceed a length of 10 m.
      2. If the problem occurs when using an ISDN hub: Connect a pre-assembled resistor to a port on the ISDN hub if the ISDN hub does not already have one, or if the ISDN hub is not terminated.
      3. Use ISDN cables that are as short as possible to connect all ISDN devices directly to the ISDN hub. Do not connect any further S0 bus extensions to the ISDN hub.
      4. If an S0 bus was laid: 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 the ISDN NT is located in the middle of your S0 bus installation, you must have termination resistors on both bus ends. In this case, you must turn off the termination in the ISDN NT.
      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 the ISDN NT

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

    Note:Your DSL provider can provide you with a new DSL splitter and ISDN NT.