Connecting to the FritzBox SIP server

There’s nothing too remarkable in what I described in the first part of this article, but in recent firmwares, the FritzBox has added a new feature – a SIP server. That’s essentially a bit of software that allows the FritzBox to appear like an internet phone company, so you can use an internet phone to make calls via the FritzBox. You could, for example install Fring on an iPhone, or use Nokia’s built in software, so that when you’re at home, you can call from your mobile phone, via your landline, and use the mobile phone to answer calls made to your landline number.

If you want this functionality, you’ll need firmware version 54.04.76 or higher. If you’ve got a fairly standard setup (ie, using the FritzBox as your internet gateway) then everything should work fairly well. Go to the ‘Telephone devices’ section of the web interface, click to configure a new device, and tell the FritzBox that it’s a telephone, then on the next screen select ‘LAN/WLAN (IP telephone)’ as the interface, and add a name.

On the next screen you’ll see a number (620 for the first VoIP phone), and you can enter a password, and the information that the ‘registrar’ is “”.

That should be all you need to set up your VoIP phone, and for most people it probably will be, because if the FritzBox is your internet gateway, it will tell devices its own IP address when they try to connect to “”

However, in my case, it just wouldn’t work. I tried resetting to factory defaults; I tried newer firmware, and I tried going back to firmware 54.04.76, which is what I’m using now. I simply couldn’t persuade my VoIP phones to register with the FritzBox.

Network setup

I suspect that at heart, this is to do with the way my network is set up. I have fixed IP addresses, and I’m not using the FritzBox as my router; I have a separate broadband router, with a more flexible firewall than the FritzBox, and the fixed IP addresses are necessary for the move to VoIP – you don’t want inbound business calls failing because your IP address has changed, after all.

So, since the FritzBox isn’t looking after the network connections, there’s nothing on my network to tell devices what they should connect to when “” is requested. Not a problem, I thought – I have a proper DNS entry that maps to the IP address of the FritzBox, so I can use that when I set things up. It works perfectly, for instance, when I want to connect to the web interface.

As I’ve discovered, however, it doesn’t work when you’re connecting to the built in SIP server in the FritzBox. Neither my VoIP phones nor the 3CX software system would register with the FritzBox server if they were set up to use the hostname; I thought I’d mistyped passwords, or needed to put ‘’ on the end of the username, all to no avail – and there’s not any decent English documentation for this new FritzBox feature.

In the end, it turns out, all I had to do was use the IP address of the FritzBox as the registrar name, and all is well, with VoIP phones connecting fine to the FritzBox, and able to use the ISDN line to make or receive calls.

FritzBox SIP client config
The Fritz!Box tells you to enter '' as your SIP registrar - but if you have problems, use the IP address instead. It worked for me

Gateway to the world

Why’s all this so important, and why am I spending my weekends playing with it? After all, if I’m running 3CX, why don’t I connect all the VoIP phones to that? Well, that’s what I’m doing – but it’s also possible to connect 3CX to the FritzBox, so that 3CX thinks the FritzBox is a VoIP phone provider, and the FritzBox thinks that 3CX is a phone.

That may seem a bit odd, but what it’s actually doing is providing me with a full two way gateway between the two devices.

It means that when a call comes in from the BT line, it can be passed into 3CX, and also – this is the main thing I wanted to achieve – that calls can be passed out of 3CX and on to the BT line. When the ISDN line is switched off, that will be the BT analogue line, which I still have to have for the ADSL service.

And since it’s there, it may as well be used. I can set up 3CX so that some 3 digit numbers go to the BT line automatically, for example. That ensures that if anyone dials 999, the call will go to a line that the emergency services can identify easily.

It’s also possible to arrange for the BT line to be used when the VoIP services via the internet aren’t working, providing me with fallback, again essential in emergencies.

You can, of course, do this with various other VoIP adaptors, some of which are cheaper than the £170 or so you’ll pay for the Fritz!Box Fon Wlan 7270, but if you want all the functionality – including the ability to work with ISDN phones too – then it’s pretty hard to beat.

7 Replies to “Connecting to the FritzBox SIP server”

  1. I would be very glad if you could help me on the configuration of my and my 3cx server. I really don’t know how to enable the gateway behaviour.

  2. I’ve not configured the FritzBox as a gateway in my 3CX server; instead, it’s effectively configured as two separate things, because the last time I looked, the 3CX gateway scripts for a FritzBox didn’t do what I wanted.

    First, the Fritz is configured as a SIP provider in 3CX, using the generic SIP provider template, so it signs in to the FritzBox as extension 620.

    That extension is configured on the FritzBox, as described above. It’s configured to send outgoing calls via the analogue line connection, and for incoming calls is set to react only to the same number.

    I have also created internet telephony accounts on the FritzBox, going in the other direction, so that it signs in to the 3CX server, as if 3CX is an external SIP provider. I have one of these set up for each extension I use on the Fritz, so four in total (entry phone, analogue phone, plus two ISDN lines).

    So, these all register with 3CX as an extension, and then are set to make outgoing calls only via their specific SIP account, and to react only to calls on that number too.

    So, everything works in both directions; effectively, 3CX and the FritzBox are both client and server to each other.

  3. I am using a Fritzbox 7390 as a SIP server. In my setup, my FB is setup to use two SIP services, one with Sipgate in the UK and one with Sipgate in Germany. A FB SIP server on extension 620 is setup to respond to calls arriving on the UK number and another FB SIP server on extension 621 is setup to respond to calls on the German number. I am also using IP addresses, instead of names.

    Everything works fine and I can even transfer calls between DECT and VOIP phones attched as FB extensions – except that the 621 SIP server only rings if it is configured to respond to all calls. The DECT phones associated with the FB do respond correctly i.e. they will only ring for phone numbers I select on the FB.

    Has anyone managed to get two or more SIP servers working correctly on the FB 7390?

  4. This would be a great functionality, but I can not get it to work.
    I am using a cloud CX3 server, would that be a problem? I checked the option: Allow registration from the Internet, but keep the error: Registration failed for: Lc:10003(@Fritzbox[]); Cause: Cause: 404 Not Found/REGISTER from xxxxxx:5060.

    Who can help me?

  5. Hello Nigel, thanks for your reply!
    I am using a fixed IP address for the Fritzbox. That should not be the problem…

  6. My installation is a little different; I don’t have fixed lines connected to the Fritzbox, but would like to use the ISDN phone connected to the Fritzbox, routing to a SIP-provider on 3CX in the cloud.
    Would that be possible in your opinion?
    Thanks, Boudewijn

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