Skype Academy Presents Cloud PBX with on premises PSTN via on premises server Pool Summary

Another in the series of the Cloud PBX technical training available on Skype Academy.

This is a summary of the Cloud PBX with on premises PSTN calling session. Name a little confusing but its covering Cloud PBX with on premises PSTN via on premises server Pool

This scenarios is where you have an existing Lync Server or SfB Server deployment deployed on premises and also have requirement to use existing PSTN connection deployed on-premises. Cloud PBX users are homed on SfB Online.

Lets go!


As always the training disclaimer


Cloud Features and functionality is subject to change and new features are continually being added so important to ensure you look out for the latest training videos. This is Jan 2017 and again presented by Thomas Binder, senior Program manager in Customer experience and deployment team


Topics for this session


Key Learning’s


Use Cloud PBX with your existing infrastructure and on premises PSTN connections which may still be in contract.

Phased transition  of moving users to Cloud PBX

Important to enable users on premises first and then move to Office 365.

What’s in scope for this training?


Similar to Cloud PBX introduction here is structure of training available and this is the session for on premises PSTN connectivity. White boxes are training sessions and blue are for structuring purposes only.

Goal to understand Cloud PBX via on premises pool




Allows you to use existing Lync / SfB server on premises to connect to the PSTN and place and receive calls.

You must have hybrid with SfB Online and share the same sip domain.

This allow you to move users seamlessly between on premises and online depending on users requirements

For Cloud PBX with on premises PSTN connectivity users will be required to be move online but the online users can leverage the on premises PSTN connection.

Signalling and media, once user is homed in SfB online all signalling for user will go to SfB online however for media will go directly to SfB on premises infrastructure.



Utilise existing environment, SBCs, PSTN contracts, PBXs

Users homed in Office 365 and leverage existing investments

Cloud PBX with PSTN calling may not be available in your region

Provides a simple migration path to SfB online and you can slowly migrate users to use Cloud PBX

Smaller hardware footprint / Less load on server as workloads on premises will move to SfB online such as signalling, meetings , IM and presence.

When to use Cloud PBX with on premises PSTN connectivity?


Touched on above but you could have users in a region where Cloud PBX with PSTN calling is not available. (Currently PSTN calling is only available in US, Puerto Rico, UK, France and Spain) more regions coming soon.

requirement to keep existing carrier contracts for example you could have a 5 year PSTN contract to preserve low call rates.

Slowly move users to Office 365 this is a great way to start and validate SfB Online and you can use existing investments on premises.



You need to meet all the requirements

  • Your existing Lync / SfB server deployment must have a SfB Edge server role deployed for external connectivity and federation enabled.
  • Front end and mediation servers can be Lync Server 2013 or SfB Server 2015
  • Server must meet Cumulative update requirement, latest update is recommended.
  • SfB need to be configured for Hybrid, more details here.
  • Active directory synchronization
  • Correct firewall config in place
  • Enterprise Voice deployed so mediation server role is deployed with PSTN connectivity fully configured.

On premises planning considerations


Mentioned above was the potential for reducing workloads and load on existing on premises infrastructure but there also potential for increasing load as well.

Reduced load can come from offloading meetings, pstn conferencing

Media by pass is not supported for Cloud PBX with on premises PSTN connectivity

PSTN calls for all users will need transcoding on the mediation server!

If you have media bypass enabled today and users are moved then there could be extra load on the mediation servers.

Service Numbers


Required for certain cloud pbx services and offers high capacity for call queues, Auto attendant and PSTN conferencing.

You can get new or port existing numbers.

You can have toll or toll free, toll means caller pays to call the number, toll free means you will pay for the call and PSTN consumption billing is required for this.

Telephone numbers – New of bring your own


New numbers are easiest method and is available via the Office 365 admin portal, user number and service numbers are available from supported countries, regions, cities.

You can bring your own numbers as well via porting process, where users can keep their existing numbers which will be moved to SfB online so no new business cards or communication of new numbers is required. More details are found in the Cloud PBX will PSTN calling session here

Consumption based billing


Very important for use of toll free numbers and important to ensure their is funds available otherwise services will not work. Funds can be one time or auto recharge, with one time funding you need to be very careful your funds don’t run out so auto recharge is recommended for safety and not to stop services.

Use for Toll free dial in conferencing, call queues or AA (CQ and AA in preview currently)

Dial out conferencing – dialling out to a country that not included in PSTN conferencing subscription

Users with Domestic PSTN calling plan that want to call internationally.

You can see that PSTN consumption although optional is used in a number of scenarios so be careful.

Call routing and call flows



Two scenarios

  • Inbound call flow
  • Outbound call flow

Inbound call flow

  • A caller calls a users the DDI / DID of the user, this use will terminate on premises PSTN gateway.
  • Call is first route to on premises SfB to check if the user is homed on-premises or online using AD lookup
  • Request is proxies out via the edge server on premises to SfB Online (using federation route, so signalling goes to SfB online)
  • From SfB all endpoints of signed in use will now ring.
  • Is the call is accepted, media is routed locally (if user is internal to the network) or through companies edge server (if user is external to your network)
  • If call rejected media is routed to Cloud PBX Voicemail (VM provided by Azure VM in Office 365)

Outbound Call Flow

  • The SfB User initiates a PSTN call
  • SfB client normalises the number based on assigned on premises dial plan
  • Client sends invite from Office 365 to On premises front end and then call is routed to PSTN



User hosted in Office 365

First client signs in and use DNS and in hybrid this point to on premises in step 2, the user will authenticate and redirect to Skype for Business online in step 3. The user signs in against Skype for Business Online. from this point forward all signalling will go to Office 365.

Peer to Peer Call Flow (Online user to online user)


Dave calls john, both users are online users, Dave types the phone number of john in his client, SfB does a reverse number lookup and finds john which the phone number and johns on SfB so don’t call via PSTN do a direct peer to peer to call. The invite is change from phone number to sip Uri. John is informed Dave is calling. Media will flow peer to peer. There are on the same network.

Peer to Peer (On premises user to Online user)


Kathy is homed on premises, she wants to call Dave, again she dial Dave’s phone number, the invite sent to FE server and again does reverse number lookup, FE discovers user is SfB online user so FE sends to edge server to Office 365 and invite is sent to Dave. Both users are internal so media will flow point to point.

Incoming PSTN call – SfB Online user on internal network


Dave receives an incoming call from PSTN to PSTN GW to FE, the FE find Dave is in SfB Online, signalling is sent to Edge Server to Office 365 and invite is sent to all endpoints. Dave answers call and media is established, media 5a flow from Dave whos local to Front end and 5b from FE to PSTN GW. Media cant flow directly from Dave to PSTN GW as media bypass is not supported with Sfb Online.

Incoming PSTN call – SfB Online user on external network (Internet User)


John is connected on the internet and connected to Office 365, incoming call from PSTN to PSTN GW, to FE to Edge server to Office 365 and invite goes to JOhn. Same process as internal user.

John answers call, John cant talk to the front end server so must speak to Edge server , to FE server to PSTN GW. If John cant talk to Edge server on premises then John may have to send traffic to Edge server in Office 365 then to the on premises Edge Server to FE to PSTN Gateway.

Media will stay as local as possible. John is external so must connected via Edge server.

Outgoing PSTN Call – SfB Online users on internal network


Dave wants to call a PSTN number, dave homed on SfB online, and signalling goes to Office 365, Office 365 attempts to do reverse number lookup if it matches a number for a user, it doesnt so it goes to edge server, front end server, FE check if dave is allowed to call this number and to check what PSTN GW to use. Media will go as direct as possible, Call goes from dave to Mediation to PSTN GW. No media bypass so media does to Mediation server first.

Outgoing call – SfB Online user on External network (Internet)


John is outside of the internal network on the internet, john want to dial a PSTN number, step 1 signalling does to Office 365, Office 365 does reverse number lookup to check if number belong to a user, If it doesnt match from Office 365 sends to Edge server, to FE to PSTN GW. For media as john is external john cant talk direct to mediation server so must first go route via the edge server to the front end to the PSTN GW.

Outgoing Call – Multi site scenario call flow – more complex


John is external to the network on the internet but we have two sites, (Redmond and Amsterdam).

Johns a US number so calls should ideally route via the US site in Redmond


Signalling is sent to Office 365 and reverse number lookup is performed


Reverse number lookup fails so signalling sent to On premises to the federation route Edge server which is Redmond in this example


Signalling sent from Edge server to Front End server, Redmond FE checks voice policies and the PSTN GW to route to.


then from front end server to Redmond PSTN gateway


Overall signalling path


Media path for the call

Media will go to the local edge server for the mediation server that is being used which is Redmond in this example, media travels direct from John to Redmond edge, from edge to mediation to PSTN Gateway.,


John now dials a Europe number


This environment is configured for all calls in europe are through amsterdam gateway.

again signalling is sent to Office 365 infrastructure for reverse number lookup which fails so its a pstn call.


Signalling sent to federation route edge server which is Redmond (NOT Amsterdam),


Edge server sends to Redmond front end server and the Redmond front end server detects the call is supposed to be routed via Amsterdam. So the Redmond front end sends the signalling to the Amsterdam front end server which has collocated mediation server role


From here the signalling goes to the Amsterdam PSTN GW


Media flow


The media will flow to the local edge server which the mediation server role is being used which is Amsterdam.

Media will travel from John to Edge in Amsterdam, to Mediation server to PSTN GW. Media direct from user to edge server.

If this flow is not direct john will send all media to Office 365 then Office 365 to On premises edge, mediation and PSTN GW.

This routing logic is configurable in your environment, where calls can go to local breakouts for least cost routing or all calls can go via a single site for example Redmond.

User enablement


How do we enable users for Cloud PBX for on premises PSTN calling, there are a number of steps that are required to be configured on premises AND online!

Recommend Deployment Workflow


Steps shown above are the recommend way, there are other ways but are not as easy. recommended to follow this.

1. Starting with configuration with PSTN ingress point.

This could be a PBX acting as PSTN gateway or an SBC or direct sip trunk into SfB. this needs to be completed only once. New offices and new locations will need to be added as required.


2. select group of SfB online user to be enabled for Cloud PBX

Important to select the right people and select the people that will benefit most from using Cloud PBX. Start with a small group and learn and expand to more users.


3. Enable users for enterprise voice and assign a DID / DDI. Can be done later but easier to do this now.


4. Assign dial plan and voice policy


5. Ensure you have Active Directory synchronisation (AAD Connect) so it ensure all the users information is replicated to Office 365 and Office 365 will be aware of these users. This is needed for reverse number lookup and which policies are assigned.

6. Now we move to action Online and assign Cloud PBX licence to the users (E3 add on or E5)


7. Move users to SfB Online (All previous steps are to be completed whilst users is homed on premises)


8. Enable for Cloud PBX and enable voicemail



Features and Class of services


Table shows features defined, dial plan and PSTN usage records with on premises and Cloud PBX. Some settings are different.

For on premises users features are defined in on premises voice policy, for online users this is defined in Online pre defined voice polices.

Dial plans are managed differently on premises or online, online dial plans are currently pre configured only until online custom dial plans are released.

PSTN usage records use both on premises voice routing policies

Actions in the Cloud


In SfB Online you need to assign policy for hosting voicemail and enable users.

Here you can see a cloud pbx licence is assigned but enterprisevoiceenabled and hostedvoicemail are both false so these need to be enabled using set-csuser.




Now they are both true.



Now users are enabled how to manage and monitor


Configuration is complete and for user management we have on premises Control Panel and Management Shell, for cloud users we can use admin portal for remote PowerShell. Most user management can be done on premises and are synced to Office 365.

Monitoring is key to understand.

  • CDR are collected on premises so you need the monitoring role deployed
  • QoE (Quality of Experience) is reported online and available in Call Quality Dashboards



  • Cloud PSTN with on premises allow you to continue using existing investments, PBX, PSTN Gateways, PSTN carriers.
  • Allows for phased transition and migration.
  • Must enable users on premises first then move to SfB Online







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