Keep it short! Why Fritz!Box, Telekom fiber optics and mobile calls are currently not such a good idea

Short connection? Then you only need to own a Fritz!Box from AVM and operate it in Deutsche Telekom’s fiber optic network (FTTH). If you then call a mobile phone number and want to talk a little longer, you will lose relatively quickly. Or rather, after just two minutes and one second. Because that’s when the connection is lost and you can (a) call again every two minutes or (b) have someone call you back. Because when the other party calls, it suddenly works.

The whole thing has occurred since the firmware update to version 7.5x and since I was pretty much the last in the rotation with my 4060, I’ve only had the error since October. Others, whose hardware was updated more quickly, had been suffering from it since July. But I didn’t know that at the time. However, I didn’t initially look for the error on the Fritz! I sent hours of error descriptions to Telekom, searched and lost a lot of time.

In the end, I had to make all calls to the mobile networks from my smartphone, which isn’t always really practical at my desk, especially as the person I was calling then also knows my mobile number instead of my office number. That’s almost coercion, but it’s unavoidable. Blanking out is not a solution either, because many people don’t even pick up the phone. And so I have a first-class support loop behind me. Gordian knot? Almost, at least.

Telekom put me on the right track when one of the more competent employees in escalation level two said that I was already the third or fourth person to call about such a problem today and that they had now been able to pinpoint the fault, which was not primarily due to Telekom, but to the faulty firmware of the Fritz! box. Interestingly, they also pointed me to the IP-Phone forum, which I hadn’t been aware of until then . And yes, there was a thread on exactly this topic.

I then recreated the whole thing and here, too, my thanks go to the network provider’s business customer service. The problem I’m facing here in a very specific context concerns communications technology, especially for calls made via certain networks and devices. It has been discovered that an error occurs when making calls to the Telekom network using VoLTE (Voice over LTE) technology. This error manifests itself in the fact that the Fritz!Box used does not recognize when the called party accepts the call. Despite the fact that the call is actually taking place, the box simply continues to “call” as if no connection had been established. Incidentally, this also happened with subscribers in the Vodafone or O2 networks.

After a precisely defined period of exactly 2 minutes and 1 second, the box ends the call with the message “not answered”. This indicates that the system incorrectly assumes that the call has not been answered. Interestingly, this specific error only occurs when calls are made to a mobile network and the called party has activated VoLTE. In earlier versions of the Fritz!Box software, especially up to version 7.3x, this problem was not present; it only seems to have occurred with the introduction of version 7.5x. This really difficult to filter out circumstance, based on a causal chain, underlines the complexity of modern communication technology, where even the smallest software changes can cause unexpected and specific problems in network communication and the helplessness of the respective support, which does not know its own product.

AVM support with two faces

I then called AVM support, as I had read that there are already so-called “lab” (beta) versions of the firmware for certain models that are supposed to fix the error. But my Fritz! box was not on the list of available devices. So let’s ask the support. That’s what I thought. The employee said it was the first time he had heard of the problem. However, I deliberately only mentioned a few minutes when describing the problem, but didn’t give an exact number. Interestingly, the employee then summarized it with Telekom, fiber optics and termination after 2 minutes. A request to please send the link to the lab version was rebuffed because there simply wasn’t one. So what? I then received another email asking me to describe my problem again, which I didn’t do. I hate time loops and endless loops to create duplicates.

A nice forum user from the aforementioned IP-Phone forum kindly sent me a download link that was not publicly available and I then carefully updated my box (save settings!). You never know… But: it worked and the first call to the mobile network took over 20 minutes! I then emailed the support team in a ticket to say that I didn’t thank them for anything, but that everything could be solved on my own initiative. However, to be fair, AVM did get back to me after all. I would also like to send you this text for the sake of completeness.

Problem solved. For me at least. However, the relevant lab version (beta) is still not in public circulation, which I think is a pity, as there are other models that have the same error. AVM should actually be able to offer at least a beta solution after more than 3 months. Because a problem like this is not only annoying, but also significantly restricts the functionality of the router.