Cloud PABX with On-premises PSTN connectivity

Sometimes my consulting engagements require creative thinking on how we can deliver Skype for Business services based on the customer needs and timing of suitable products becoming available to the market. In this case my customer wanted Skype for Business online with enterprise voice with  Telstra calling for Office 365. At the time the Telstra PSTN calling plan was not generally available. Business issues and time constraints required the business to implement a new greenfield solution within a week. Normal Telco lead times and gateway acquisitions can take four to six weeks to have SIP infrastructure ready. Gateway acquisitions can be expensive, especially if it becomes redundant when the customer moves to a full cloud solution as soon as Telstra PSTN calling plans for SFB are available.
In this design I implemented a hybrid solution using a new Skype for Business Server deployment and PSTN connectivity through a third-party SIP trunk provider for Skype for Business. Through this provider we could purchase PSTN numbers and connectivity without the need for a hardware gateway appliance. The solution required a hybrid topology. The initial implementation required an on-premise solution with a single Skype for Business front end server and an SFB edge.
sfb hybrib
The users are initially homed on-premise and PSTN delivered through 3rd party SIP trunks terminating onto the mediation service role. The PSTN media is anchored through a registered public IP that the Telco provider allows. On the Skype for Business server the 3rd party SIP hosting service is configured as a standard PSTN gateway
To take advantage of Microsoft Cloud PABX features we can simply migrate the on-premise user to the cloud. In this topology, users are homed in the cloud on Skype for Business Online instead of being homed on the on-premise deployment.
With this option, your Skype for Business Online users get their Enterprise Voice PSTN connectivity through  the  on-premises Skype for Business Server deployment.
So how does Cloud PABX know to associate the on-premise PSTN with the user?
Through the Office 365 online power shell portal we can look at the users online properties. The get-csonlineuser command needs to show on-prem enterprise-voice is enabled, a SIP address and the on-Premises line URI as in the example below.
Next in the on-premise SFB Management Shell I run the get-csuser command to retrieve the users on-premise properties and find the user is assigned the Voice routing Policy of Global-Hybrid. I then run the get-csvoiceroutingpolicy command to check the Global-Hybrid voice routing policy and determine the PSTN usages assigned to the user. The PSTN usage configuration in the on-premise Server will determine the route used to dial out.
The cloud PABX user and a on-premise SFB user in this SFB hybrid scenario will both  follow the on-premise call routing logic. The PSTN usage configuration in the on-premise server will send the call to the 3rd Party SIP trunk provider.
Note: Telstra calling for Office 365 with Skype for Business is now available. This will allow a PSTN calling solution without any on-premises infrastructure.

Removing blockers for Cloud PABX with On-Premise PSTN for Skype for Business Online.

Overcoming obstacles to migrating to Cloud based enterprise voice solutions is achievable through clever design options. Some enterprise business infrastructure managers may feel that their legacy voice environment is restricting the migration of voice services to cloud based offerings like Skype for Business or Microsoft teams. However, Microsoft offers a variety of design options for enabling PSTN connectivity for Office 365 accounts or Skype for Business accounts with your on-premise Skype for Business Server deployment. Microsoft Cloud PBX with PSTN access through a Hybrid Skype for Business on-premise deployment can provide a migration strategic vision.
Your goal maybe a full unified communications cloud vision, for example Skype for Business Cloud PABX with Telstra calling for Office 365 plans or a hybrid, but your organization migration strategy may be blocked. This issue may arise for some of the following reasons:

  1. Locked in TELCO contract or Investments in On-Premises PSTN connectivity
  2. Legacy PABX infrastructure capital investment.
  3. Existing On-premise server applications ie: contact centre and call recording Solutions.
  4. Dependency on analogue infrastructure

With a Hybrid SFB solution It is possible to realise a staged migration to Hosted Cloud PABX services now. Therefore, your migration strategy may be migrating some of the user base now and testing cloud-based telephony until the restricting forces are removed.
In this scenario you can move users to Skype for Business Online Cloud PBX with SFB on-premise PSTN connectivity.
Skype for Business Server Hybrid deployment
sfb hybrib
The diagram shows a hybrid design that consists of a Skype for Business Server on-premise deployment federated with Office 365. Users can be homed on premises or online in cloud. The users will be able to make and receive calls through existing on-premises voice infrastructure. Notice that existing legacy PABX infrastructure is integrated into the solution. Additional third party gateways from Microsoft qualified gateway provider can deliver multiple connectivity options for legacy infrastructure to integrate into a SFB solution.
Overcoming the barriers for migration to Skype for Business:

  1. The business can maintain existing TELCO PSTN contracts. Existing PSTN services can terminate on SFB session border controllers that are controlled through the on-premise server. The business can then simply port the existing number range to new Office 365 Telstra  PSTN plans when existing contracts expire.
  1. Integration with legacy PABX solutions can be maintained through SIP or ISDN Links between SFB and 3rd party PABX solutions through a Session Border controller. This can maintain investments in legacy services and also enjoy benefits of Microsoft Cloud PABX and digital workspace.
  1. Key users on legacy contact centre and call recording applications can remain on existing platforms. Cloud based contact centre solutions can be evaluated and migration strategies separating contact center users from the rest of the business can be created.
  1. Legacy analogue requirements may be maintained through SFB on-premise servers and provision of analogue gateways

Therefore, if your business objective is progressing to Cloud based enterprise voice infrastructure and committed to Office 365 Cloud PABX as your voice solution then you have migration strategies with a SFB Hybrid design.
Keith Coutlemanis

Cloud PABX

Are you looking at a Skype for Business Cloud solution and somewhat perplexed by the many options available? It is confusing choosing from the types of Skype For Business licensing and topologies however you will need to contemplate the following concepts to help you make a decision on the migration path to Skype for Business Cloud PABX.
Cloud PBX
is your phone system in the cloud, it is tightly integrated with Office 365. With the E5 license users get the voice capabilities they need—make, receive, and manage calls from anywhere, using any device. You have the choice to use your current telco provider or subscribe to Microsoft PSTN calling for public call access. Cloud PBX also provides great controls for IT to increase agility and consolidate management compared to traditional phone hardware.
PSTN Calling is an add-on to Cloud PBX that provides domestic and international calling services for businesses directly from Office 365. Instead of contracting with a traditional telephony carrier and using an on-premises IP-PBX, you can purchase the Cloud PBX from Microsoft and add on PSTN Calling for a complete enterprise telephony experience for end-user. With PSTN calling Microsoft is the Telco provider. However, PSTN Calling subscription will be a few months before it is released in Australia.
Cloud PABX Cloud Connector Edition is available now in Australia. So, ask yourself the following to see which is right for you. Some common questions customers need to ask

Existing Contracts

Do I have existing long-term contract with PSTN provider?

  • If you are locked into a telco provider you may be receiving a generous discount for all your PSTN, MOBILES AND DATA services. Microsoft Skype for Business Cloud Connect edition is mechanism to extend your telco services to the Cloud through an on-premise gateway. It is possible to maintain existing telco agreements with this topology.

Analogue Devices

Do I have lots of Analogue services and Common Area Phones?

  • One of the biggest challenges with migrating to Cloud PABX is what are you going to do with all your old legacy analogue devices. Ideally you will replace these devices with Skype for Business optimised devices. However, you may encounter a road block that will stop you upgrading these devices to new IP phones. For example, that door phone and release relay, what do you do with that? Or the phones in remote warehouse locations not covered by a data network.
  • The legacy fax service can be migrated to hosted fax services or multifunction devices.

You have two options:

  1. Replace legacy Analogue devices with low end LYNC online IP phones and pay a basic SFB online subscription.
  2. Deploy SFB Cloud PABX CCE and configure the on-premise gateway with additional Analogue services and configure special routing tables for it. The following diagram shows how legacy infrastructure is integrated.


Consider the following before implementing a Cloud PBX


  • One portal in Office 365 for email and phone system
  • Easy to setup and maintenance
  • Minimal on-premise Infrastructure upgrades needed
  • Lowest expertise needed for use and maintenance
  • Lower TCO compared to on-premise Skype for Business implementations
  • Minimal Incremental license addition to enable E5
  • Expensive when compared to third party hosting
  • Missing feature parity compared to on-premise Skype for Business versions
  • Minimal control over upgrades and customization

You have invested significant capital in a legacy PABX network. The PABX vendor is charging you software subscription and upgrade fees to keep your legacy telephone infrastructure current, your maintenance contracts costs are high and all you are getting is the ability to make and receive voice calls. If you subscribe to Office 365 E3 then it is a small incremental cost to update to a Cloud PABX E5 license. The business drivers to migrate the whole network to a cloud based model may result in substantial cost savings.

So, you have your business driver to migrate some or all of your PABX infrastructure to Skype for Business Cloud PABX and with the correct integration topology it is possible to perform a staged or full migration. I will describe this in my next Blog.

Keith Coutlemanis

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