When I am talking on a FRITZ!Box extension over my ISDN line, and another call comes in, the caller does not get a busy signal. How can I change that?

If someone calls you while you are talking over the ISDN line, the FRITZ!Box settings and your ISDN equipment's configuration determine whether the caller hears a busy signal or a ring signal. If you want callers to hear busy signal when you are on the phone, configure your equipment as follows:

Configuring the FRITZ!Box

  1. Open the FRITZ!Box user interface.
  2. Select "Telephony > Extensions"
    or
    "Telephony " > "Telephony Devices"
  3. Click on the "Edit" icon next to the extension you wish to configure.
  4. Select the tab marked "Extension Properties" or "Telephony Device Features".
  5. For all extensions that you use, activate the option "Call rejection on busy (Busy on Busy)".
  6. Click "Apply".
  7. For all extensions that have no analog terminal equipment connected:
    • Select the "Telephone" or "Extension" tab.
    • Delete any numbers in the "Number of the extension" and "Additional numbers" or "Outgoing calls" fields.
    • Deactivate the option "React to all numbers" and click "Apply".
  8. If you have connected ISDN terminal equipment to FRITZ!Box's "FON S0" port, activate the "busy on busy" setting in your ISDN devices as well. For instructions on configuring your ISDN terminal equipment, consult the pertinent owner's manuals.
  9. Assign your ISDN terminal equipment on FRITZ!Box's "FON S0" port different numbers for incoming call acceptance (also called incoming or dial-in numbers) from those used by the analog FRITZ!Box extensions.