The following offerings are available;

  • Cloud PBX with PSTN Calling
  • Cloud PBX with on premise PSTN connectivity
    • Via an existing Lync/Skype for Business Server pool
    • Via Cloud Connector Edition
  • Cloud PBX PSTN conferencing

It is important to note that initially, the service does not provide all the functionality that on premise counterpart does, to understand what you get with Cloud PBX, click here

Cloud PBX with PSTN calling

  • This offering allows customers, to utilize Microsoft as their sole telephony provider
  • User accounts are homed in the cloud, telephony services are hosted in the cloud, under a financially backed SLA
  • Either acquire new telephone number ranges for organisation, or port your existing telephone numbers into the service and have no dependencies for on premise server infrastructure to deliver your telephony services
  • Cloud PBX PSTN Calling is not available in all geographical locations, although there are deployment considerations that can be factored in to easily overcome this hurdle

For customers interested, it is perfectly viable to pilot the service, acquiring telephone numbers for users and allowing for evaluation of the service in line with preview program updates. This is a clean, non-disruptive way of testing out the Cloud PBX with PSTN calling functionality.

Whilst there is not feature parity with the on-premise service, to account for this Microsoft have presented several ways for customers to achieve the Cloud PBX functionality, each of those will be discussed now.

To enable a user, you must;

  • Acquire or already own an Office 365 tenant with Cloud PBX add on license included, click here for more information
  • Acquire new or port existing telephone numbers
  • Assign a Skype for Business Online add on license to the users
  • Acquire and assign a Skype for Business Online PSTN Calling Plan to end users
    • PSTN Calling Plans provide minutes to end users for outbound calling. National (domestic) and International calling plans are available
  • Assign a telephone number to the user from the available pool

There are additional considerations to be considered, including but not limited to Office 365 Directory Synchronization, ADFS, End user devices, Office 365 Express route and networking planning, dependent upon your business and technical requirements.

Cloud PBX with on-premise PSTN Connectivity

Falling in line with the hybrid topologies, that allow organizations to transition services over time, Skype for Business Online Cloud PBX supports this platform topology.

As stated earlier, the current cloud PBX service from Microsoft does not provide all the functionality that is available from an on-premise deployment. To account for this, Microsoft allow for the retention of existing telephony carrier relationships and on-premise deployment applications such as contact centers, through the hybrid topology.

For those users that require basic corporate telephony capabilities, home them in the cloud, for users requiring access to custom ISV applications, they can remain on premise, whilst the organisation continues to integrate new functionality as it arrives within the online service (Trusted Application Online API now in preview)

This offering breaks down into two further offerings;

  • Cloud PBX with on-premise Skype BusinessLync Server
  • Cloud PBX without any existing on-premise Skype for business Lync

Cloud PBX with on-premise via an existing Skype for BusinessLync Server

This option is like the Hybrid Voice topology that was previously released and withdrawn, to and from the market.

It allows organisations to leverage the existing investment made into building out an on-premise deployment with a view to transitioning all services to the cloud over time.

In this scenario, some users are homed on premise, some users are homed within Skype for Business Online, but the voice services required by all users are delivered via the Skype for Business Server (on premise) infrastructure and existing billing relationships. CCE does not actually homed users, it provides a mechanism for routing traffic to and from Skype for Business Online to end users.

Important notes to consider during planning:

  • To enable an SfBOnline user for this Cloud PBX offering, you must have provisioned your company domain (e.g. to your Office 365 tenant
    • domains are not supported
  • Lync Phone Edition must be updated to the minimum required firmware

System Requirements / Prerequisites

  • Front End Server must be running Skype for Business Server 2015 or Lynch Server 2013
  • Edge Server must be running Skype for Business Server 2015 or Lynch Server 2013
  • Mediation Server must be running Skype for Business Server 2015 or Lynch Server 2013
  • Enterprise Voice is configured and tested on premise, including all PSTN components; SBCs, IP-PBXs, PSTN Gateways…
  • Azure AD Connect
  • Hybrid Connectivity must be enabled between your on-premise deployment and Office 365 tenant
  • To support Single Sign On for end users, Active Directory Federation Services must be provisioned

Cloud PBX with on premise connectivity, without Skype for Business Server (Cloud Connector):

This scenario applies to organisations that have not yet deployed any Skype for Business or Lync infrastructure, but wish to adopt Skype for Business Online for all Unified Communication and Telephony functionality whilst retaining existing telephony circuits and billing relationships.

Whilst not currently supported, the roadmap for CCE allows for it to be deployed alongside a Skype for Business Server pool. What this means is that for the geographical locations that the Microsoft PSTN Calling Service is not currently available in, CCE is a deployment option allowing for local SIPISDN trunks to be acquired.

This appliance, is used to create a SIP trunk connection to a supported PBX or SBC appliance, which becomes the gateway for the online homed user account voice traffic. Users are homed online and consume UC services via the online pool, whilst PSTN voice traffic is routed via the Cloud Connector VMs via the existing telephony infrastructure.

In this instance, organisations must deploy a small ‘flavor’ of an SfB deployment called ‘Cloud Connector Edition’. This virtual appliance server is a virtual machine that consists of the following server roles:

  • Central Management Store (CMS) Role
    • Configuration store for the topology components
  • Edge
    • Access Edge
      • SIP Routing between on premise and online services
    • Media Relay and Media Daly Authentication
      • Media routing and authentication token for media routing
    • Outbound Routing
      • Supports only global policies based on outbound PSTN numbers
    • CMS Replica
      • Maintains a copy of the CMS local and synchronizes data from the Global CMS
  • Mediation Server
    • SIP and Media gateway between Skype for Business and the on premise PSTN gateways
    • Includes as CMS replica

System Requirements / Prerequisites

  • domains are not supported
  • Cloud connector edition is currently supported on Hyper V hosts
  • Cloud connector is provisioned using PowerShell scripts that may change the configuration of your Hyper V Hosts – review them!
  • CMS and Mediation roles can be collected on a single Hyper V Host
  • Edge Server VM must be provisioned on a separate Hyper V hosts that is deployed into a DMZ
  • Administrator permissions over the Hyper V Host
  • Administrator permission to publish the topology in the on-premise domain
    • AD Schema
    • Enterprise Admin
    • Domain Admin
  • External DNS Records
    • ap.<Domain Name>
    • mr.<Domain Name>
  • Your Office 365 tenant must have the required SRV records created for it
  • External Edge Certificates must be procured
  • Firewall ports 443, 5061 and 3478


Due to the state of play that we are currently in with Cloud PBX, risual do recommend getting in touch to discuss and evaluate your requirements further with a view to recommending a solution based upon your needs.

Cloud PBX is here, it is available and many customers are now enabling the service for end users and making that transition. Cloud PBX offers a rich set of client based functionality, and Microsoft have recently overlay this with Call Queuing and Attendant functionality, making it a strong contender for small businesses and even larger organisations with less complex requirements. risual can provide a Cloud PBX PoC offering that enables organisations to understand the capabilities of the service in a working production environment.

As it stands today, anybody wanting to leverage functionality such Skills based Contact Centre Routing, Call Recording and Compliance PBX functionality offered by Skype for Business, must deploy the on-premise iteration of the product, Skype for Business Server (or a hosted service via a third party). This enables customers to migrate all telephony functionality into the new server infrastructure in a smooth, well-structured approach when planned correctly.

In addition to this, Microsoft have released the Trusted Application API preview, for which Partners and ISVs now have access to ensuring that the Cloud Based Voice Application catalog is updated with the functionality you would expect to see delivered in an enterprise PBX solution, including but not limited to Contact Centre, Meeting Management, Call Recording, Compliance functionality and the much anticipated BOT functionality.