FRITZ!Box 7390 Service - Knowledge Base

FRITZ!Box 7390 Service
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Line goes dead after several rings when making calls with an ISDN telephone

When attempting to use an ISDN telephone connected to the FRITZ!Box to make calls from your landline, the line goes dead after several rings.


  • The ISDN telephone cannot 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 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 #".

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 (lift the receiver).
  2. Hang up again after about 3 seconds.

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

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