Fritz!Box 7490 with fiber connection, routing anonymous/withheld ISDN calls to a new answering machine

I got a bit fed-up with truckloads of anonymous calls. 95% of these are from sales persons. The others from organisations shielding their phone numbers. Too bad for the latter: if they want to stay anonymous, they’re not worth dealing with.

My original setup was incoming ISDN connected through a Gigaset DX600A with a bunch of DECT handsets, but there you need to configure each handheld device and the Gigaset base station itself to skip anonymous incoming calls.

One of my fiber connections at home goes through a Fritz!Box 7490 which has a connector to provide an internal ISDN S0 bus. I discovered – though not advertised in the German or English manual – that you can hook it to external ISDN even though it is not connected to external DSL but external fiber to the home (FTTH).

A note first: the the Fritz!Box 7490 has built-in resistors to terminate the S0 bus, so it can only be on one side of the S0 bus. The other far end of the S0 bus needs to be terminated too.

Steps to hook up ISDN on both incoming and outgoing S0 bus:

1. 

   Fritz!Box Y-Cable for connecting PSTN or ISDN (and optionally DSL) to your modem.

   Put the long end of the Y-cable ** into the DSL/TEL connection of the Fritz!Box.
   The long end is usually labelled DSL/TEL.

2. Put the black small end into your ISDN NT1 (sometimes called ISDN NTBA).
   The black small end is usually labelled TEL.
3. Leave the grey small end disconnected. It is usually called DSL. Since I’m using FTTH, no DSL for me.
4. Connect the chain of other ISDN devices (and the terminator) to the FON S0 connector on the Fritz!Box **.
5. Test if your original equipment still works.

Configuring your Fritz!Box to use ISDN MSNs:

1. Logon as administrator to your Fritz!Box.
2. In the menu on the left, click on “Telephony”, then on “Telephone Numbers”.
3. A new View appears, where you click on the “Line Settings” tab.
4. Ensure “Landline network enabled” is enabled, then click on “Apply” and wait for the changes to be saved.
5. In the menu on the left, click on “Telephony”, then on “Telephone Numbers”.
6. Click on “New Telephone Number”, then follow the steps in the wizard:
   1. Choose “Configure landline number”, click “Next”
   2. Choose “multiple landline numbers (ISDN line)”, click “Next”
   3. Add the MSN numbers you will use in your Fritz!Box each in a separate entry, then click “Next”
   4. Click “Next” to save the settings.
7. Configure any devices you need to connect to the MSN numbers you entered.

Setting up the diversion of Anonymous calls to an Answering Machine:

1. Finally I set-up an answering machine for anonymous call forwarding:
   Automatically diverting anonymous calls to the internal answering machine | FRITZ!Box 7490 | AVM International.
   • Note that here you can divert all anonymous calls to “all numbers”, but not divert all anonymous calls to “Landline Network”. I think this is a bug in FRITZ!OS 6.20.
2. And (since the Fritz!Box already has some VOIP numbers configured), I also added the GigaSet as an ISDN PBX to the Fritz!Box:
   1. In the menu on the left, click on “Telephony”, then on “Telephone Devices”.
   2. Choose “Configure New Device”, then follow the Wizard Steps:
      1. Choose “ISDN PBX”, click “Next”
      2. Note the MSNs the Fritz!Box shows (they should include both the ISDN MSNs and VOIP numbers), then click “Next”.
      3. Verify the ISDN PBX (the GigaSet) is on the FON S0 port, then click Apply to confirm.
   3. In the menu on the left, click on “Telephony”, then on “Telephone Devices”.
   4. Choose “Edit” for the “ISDN PBX” connected to “FON S0”.
   5. Verify the “Main Phone Number” is correct (choose either an ISDN MSN or VOIP number).
   6. Then I configured the extra VOIP numbers as MSNs in the GigaSet.

–jeroen

** Larger images.

Large picture of my setup (I found this later while researching images, thanks Rob Kalkmeijer for posting this one):

Fritz!Box connected externally and internally to an ISDN S0 bus.

The connectors on the back-side of my Fritz!Box 7490:

Fritz!Box 7490 connectors.

Large picture of the Y-cable:

Large picture of the Y-cable. Many show the DSL, TEL and DSL/TEL on the various connectors.