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Knowledge document #94
FRITZ!Box 7590
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Outgoing calls with ISDN telephone hang up after several rings

When you make landline calls with an ISDN telephone connected to the FRITZ!Box, it rings several times and then hangs up.


  • The ISDN telephone does not correctly process the signal for the "call-back on busy / no response" function that the line being called sends to the telephone.

1 Preparations

  1. Configure the telephone so you can send keypad sequences (sequences of characters 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 #".

2 Disabling "call-back on busy / no response" in the FRITZ!Box

The ISDN telephone terminates the connection because the signal for the "call back on busy / no response" function is not processed correctly. Therefore, disable the use of this function in the FRITZ!Box:

  1. Enter #960*4* on your telephone and press the call key (pick up the handset).
  2. Hang up again after about 3 seconds.

    Note:You can permanently reenable the function "call-back on busy / no response" by entering #960*6*.

You can still use the "call-back on busy / no response" function if required; to do so, press the call key again after hanging up and enter *37#.