Along with emailing and chatting, telephoning is one of the most important communication tools of our time. Even more, long before we could write emails or chat, the telephone was one of the communication tools that kept our lives and our economy alive, along with the fax.

In the past years, especially during the pandemic, not only the technology has evolved. But also the way telephony can serve us.

In this article, I describe how telephony has evolved and why it makes perfect sense to migrate to Microsoft Teams telephony as soon as possible.


Conventional ISDN telephony.

Who doesn’t know it, in the datacenter there is always a rack with mostly green LAN cables or a separate cabinet on the wall with thin copper cables which are closed in on themselves like a mini universe. Often they say: “We don’t have to take care of that, our telephony partner will take care of that”.
Often, a dedicated line had to be pulled into each location for the telephony solution and the power supply was also solved separately.

To help us break this down a little better, I have created a small graphic that shows us in a simplified way how a conventional ISDN telephony system used to work:


Let’s take a closer look at this simplified diagram: First we see the propriotary ISDN telephones and fax machines. In order to be able to operate these, a corresponding ISDN socket had to be available in every room. The great advantage of ISDN telephones at that time was that, in contrast to analogue telephony, several calls could be made at the same time. Calls could also be forwarded. But then the flexibility was already reached. The end-users were tied to the location and could not simply work from another place without a certain amount of effort.

An ISDN connection is also called a “false RJ45 connection”. This is because it looks quite similar to a classic RJ45 socket, but the PINS are assigned differently.

All ISDN telephony then converges in a so-called telephony rack and is routed via one or more ISDN business lines to the PSTN telephony network.


VoIP telephony

The next evolutionary step after ISDN telephony was VoIP telephony. This was a quantum step forward in various areas. On the one hand, existing ISDN hardware could continue to be used in part thanks to a VoIP adapter. On the other hand, the option of softphones was added. It was no longer necessary to have a physical device on the table. Various applications for computers could take over these functions. Thus, from this point on, “wrong RJ45 sockets” could be dispensed with. Also, the LAN cables that are specially available for voice are no longer necessary.
Here is a simplified diagram of how VoIP usually works:

VoIP is still widely used and sold today, but let’s take a step back and answer the following question:
What exactly is VoIP?
IP telephony (short for Internet Protocol Telephony), also called Internet telephony or Voice over IP (VoIP for short), is telephoning via computer networks that are built according to Internet standards. Information typical for telephony, i.e. voice and control information, for example for establishing a connection, is transmitted via a network that can also be used for data transmission. The call participants can be connected via computers, telephone terminals specialised in IP telephony or classic telephones connected via special adapters.
IP telephony is a technology that makes it possible to realise the telephone service on IP infrastructure so that it can replace the conventional telephone technology including ISDN and all components. The objective is to reduce costs by means of a uniformly constructed and operated network. Due to the long period of use of conventional telephony systems and the necessary new investments for IP telephony, the change is often realised as a long-lasting, smooth transition for existing providers. Meanwhile, both technologies exist in parallel (smooth migration). This results in a clear need for solutions to connect both telephony systems (e.g. via VoIP gateways) and the necessity for targeted planning of the system change, taking into account the respective possibilities for cost and performance optimisation.


Cloud telephony with Microsoft Teams

As we could read up to here, a VoIP system is actually already really good, so why switch to Microsoft Teams telephony now? – After all, it’s “only” VoIP too…
There are several reasons, I will try to explain the most important ones here:

Microsoft Teams is already used by many as a collaboration solution. Microsoft Teams is the follow-up solution of Skype for Business and will also play a central role in the new Windows 11 operating system.
Many of us already use Teams and other Microsoft 365 solutions in everyday life. Chatting, internal team calls, video conferencing, Exchange online and Share Point online integration are already implemented in many places.

With the desktop version under Windows 10 and the integration in Windows 11, there is no need for additional software to be installed and maintained. Any Android or iOS user can easily download the Teams application from the store, log in with their company credentials and start working immediately.
This option in particular makes us more flexible. For example, we can take a call on our computer and easily transfer it to our mobile. This feature is especially appreciated by employees who travel to and from work by public transport.

From the administrator’s point of view, it is also easier to maintain physical Teams phones. This is because all devices are visible in the tenant and updates can be managed from there.

But the most important points from my point of view, why a migration to Microsoft Teams telephony makes sense, can be seen in the following graphic:

As we can see very well in this graphic, no dedicated hardware is needed for Microsoft Teams telephony. Even the Teams certified physical phones and rooms are 100% LAN capable and can be easily integrated into the existing LAN.
Once Teams is configured, only an internet connection is required for the Teams application or physical device to log on to the tenant.
Even in the event of a power failure or problems with the business internet connection, the mobile device can be used in most cases and calls can be made through the Teams app installed from the App Store.

Another advantage is that the Microsoft tenant, which is high available, is directly connected to a SIP provider. This SIP provider has a (hopefully) high available Session Board Controller (SBC) which is responsible for the SIP Trunk and the connection to the PSTN network.
Of course, it is also possible to operate your own session board controller with Teams Telefony. From my experience, however, this makes not much sense.

Alternatively, in several countries it is possible to obtain the SIP Trunk and the tasks of the SIP provider directly from Microsoft. You can find out in which countries this is already possible HERE.

If we now think about what we have read, we can also see the following:

A user is absolutely independent of location. Especially with the current pandemic, where home offices are mandatory or recommended in many countries around the world, this point is extremely helpful. Since we do not need the company network to use Teams as a telephony solution, we can now work independently of location.

After I have explained these points, the question most people ask is: “But what about ring calls?
Practically every company has at least one ring call, be it the main number or team number or, as another example, the helpdesk number.

These queues and ring calls can be easily registered in the Teams Admin Center. Users and/or groups can then be assigned to this queue. Furthermore, it is possible to allow users to log out of the corresponding queue or not.

So if a user is registered in the call queue and someone calls the corresponding number, it rings at this person – regardless of where this person is located.

Of course, different configurations can be made. We have the following options:

Attendant routing – When attendant routing is used, the first call in the queue will ring all of the call agents at the same time. The first call agent to pick up the call gets the call.
Serial routing – When serial routing is used, incoming calls will ring call agents one by one, starting from the beginning of the call agent list.
Round robin – When round robin routing is used, each call agent will get the same number of calls from the queue.
Longest idle – When longest idle routing is used, the next call in the queue will ring the opted-in call agent that has been in presence state Available the longest.

Furthermore, in addition to the aforementioned settings, we also have the option of activating the “Conference mode” (this means: This controls how calls are connected to call agents. When you turn this off, a call is connected to a call agent using a traditional transfer. If you select Auto and the caller and call agent support it, a call is connected to a call agent much faster than if it uses a traditional transfer.)

and/or the Presence based routing (When turned off, calls will be routed to agents who have opted in to receive calls, regardless of their presence state. When turned on, opted-in agents will receive calls only when their presence state is Available.)



As you can see, migrating to Teams Telephony makes sense on several levels. Of course, there are many more options that can be configured, besides the text-to-speech option where the recording of greetings will be omitted in the future, the configuration of working hours and hollidays simplifies the operation considerably. There is also practically no maintenance work on the telephony infrastructure. The only thing that needs to be done is the update of the physical certified devices. However, these can be carried out centrally from the Admin Portal.

Finally, there is the question of licensing for the telephony option at Microsoft.
It should be noted that in addition to the normal Teams licence, a Phone System licence and, depending on the license, an Audio Conferencing licence are also required. The phone system licence must either be purchased per user or it is included in diverse plans such as Office 365 E5 or Microsoft 365 E5.

Furthermore, depending on the SIP provider, you have to take into account what is required for a calling plan. If you decide to host everything at Microsoft, we are talking about either a Domestic Calling Plan or Domestic + International Calling Plan. These can be purchased as a flat rate or by the minute tariff.

Yes, this sounds like a lot of extra costs, but please note that you don’t have to operate your own telephone exchange, which normally costs money for operation, maintenance and licences.



There are also other licences that need to be considered, so here is a Postskriptum. For central numbers such as a helpdesk number and the main number that are not assigned to a physical user, a Phone System Virtual User licence is required. These are partly free of charge. You can read exactly how this works HERE in the official Microsoft link.