With the Cloud PBX (Private Branch Exchange) with PSTN call, you can connect your business to the Public Switched Telephony Network (PSTN) and give your users a primary phone number in Skype for Business, so they can make and receive calls.

A cloud PBX system is based on cloud computing technology where data is stored and transmitted over the Internet, rather than on a computer or hardware owned by the end user. Cloud technology that has been around for many years but has only recently become popular for end consumers with the introduction of programs such as MobileMe and iCloud, both consumer-based cloud computing technologies.


Users assigned phone numbers in Skype for Business Online can use Skype for Business IP phones, PCs and mobile devices to make and receive calls.

PBX, or Public Branch Exchange, is the name used to refer to the technology with which a phone provider routes the calls. Originally, a PBX was a large unit of hardware that had to be stored on-site and operated manually. These machines could be very costly for a company because they were large, required special storage space, required regular repairs and had to be operated manually.

Companies with multiple phones and many employees cannot function properly without office PBX technology. In recent years, a telephone system could be designed to work automatically and without an operator, but even these devices were expensive and complicated (the cheapest cost about $400) and when a company moves to a new location or needs to add or remove a telephone line, the process can be very costly and time-consuming.

Microsoft's Cloud PBX has been available for some time as part of the Office 365 Cloud subscription. The new features have increasingly enhanced Skype for Business to position itself in the market as a fully-fledged PBX. Of course, it is also possible to establish a connection to the public telephone network so that calls with other telephones and mobile phones can be made directly from the Skype for Business Client.


To get these phone numbers for users, you can search and reserve phone numbers in Office 365 or transfer your existing phone numbers from another service provider or carrier.

A PBX is a telephone system for enterprises. The Add-On Phone System in Office 365 offers you PBX features, but without the complicated and expensive devices. With the PBX add-on you can use Skype for Business to perform basic tasks such as making and receiving calls, forwarding calls and muting or disabling calls from almost anywhere with Internet access. It's available worldwide.

There is some confusion and uncertainty about what options are available with the Cloud PBX if you want to connect it to the PSTN. This has not been a problem with an existing local Skype for Business installation. All the hardware and servers you need are run locally and the local connection to the PSTN is through a provider of your choice.

But the trend demands that everything (or as much as possible) should be possible in the cloud without local infrastructure. So, if you not only want to use Skype for Business Online as a fully-fledged UC solution, but also want to benefit from the cloud PBX, you won't be able to do it without a PSTN connection.

The challenge is therefore to connect the cloud telephone system to the PSTN. For this reason, Microsoft has published three different types of connection to the PSTN for the cloud PBX, one is cloud-based and two use the local network.

1. Cloud-Based PSTN-Call

With this option, Microsoft manages all users in the Office 365 Cloud and makes the PSTN service available as a provider. In other words, no local servers are necessary. The entire infrastructure is provided as a complete cloud solution in the familiar Microsoft cloud. With this solution, Microsoft is a fully-fledged provider that enables the provision of public telephone numbers in just a few steps. The user receives everything from a single source and can make calls from all Skype for Business devices without having to conclude additional contracts with other telephone providers. This feature is known as a PSTN call and is available as an add-on package to the Cloud PBX.

2. Cloud PBX with a Local Connection

This option is used if you are using a cloud PBX and have an existing local PBX or gateway, but PSTN calls are not possible in the cloud. In this case your local PSTN can be deployed using the cloud connector.

The users are in the cloud PBX and are also managed there. The cloud connector offers you a range of packaged virtual machines that offer a minimum of Skype for business server topology: Domain controller, CMS (Central Management Store), Edge and Mediation Role. These are operated on-site at the customer's place. It should also be mentioned that the domain controller does not replace your existing controller but is used to communicate with the Office 365 cloud. The cloud connector is available worldwide and can be downloaded directly from the Microsoft Download Center.


3. Cloud PBX with Local Skype for Business Instance

The third and final option is for following scenario: You already have an existing local Skype for Business infrastructure with a connection to the PSTN and plan to move all or some of your users to the cloud PBX. You want to allow cloud users to use the existing local PSTN. The idea behind this solution is that you decide how quickly and when to migrate which users to the cloud. This can be beneficial if you have users connected to a solution that requests a local instance of Skype for Business. So, you can migrate all unaffected users to the cloud and leave the "special cases" in place. Once the users are in the cloud, the familiar telephony features will continue to work, as will the connection to the local PSTN, under the circumstance that the local instance is active.


Transferring phone numbers to Office 365

It's easy to transfer your phone numbers from your current service provider to Skype for Business. After you import your phone numbers on Skype for Business, Microsoft becomes your service provider and charges you for these numbers. If you want to use the existing phone numbers, you will need to create a "Port Order". Here is the Microsoft article about creating the Port.