Configuring PSTN Failover for Skype certified IP Phones with Ribbon SBC1000

Whilst the industry momentum is going towards adopting Microsoft Teams as the successor to Skype for Business, there will still be organizations that have yet to fully migrate to Teams and may continue to use Skype for Business for the interim term, particularly for companies that have invested in and leveraging Microsoft Phone System with SFB Online and hybrid voice. Whether its using CCE or Skype On-Prem Cloud Hybrid, their users are able to make PSTN calls via a local SBC with the associated SIP Trunk or E1/T1 lines. For these environments, it is desirable to retain PSTN calling in the event that the WAN connection to Office365 becomes unavailable. In this blog post, we walk through the steps on how to configure a Ribbon SBC1000 for PSTN failover such that if the WAN connection to O365 is disrupted, Skype certified IP Phones will lose connectivity to SFB Online but yet still be able to make PSTN calls via a secondary SIP connection to the SBC

Important: The Poly CCX500 used in this lab is both a certified Teams phone AND a certified Skype for Business Phone. In this lab we are using it as a Skype for Business Phone to show PSTN failover capability. This PSTN failover capability is NOT supported in Teams mode at the time of this blog post.

When using the Poly CCX500 Teams phone in a Skype for Business Online environment, it functions almost identical to its predecessors of the Polycom VVX family of phones. We login on the phone using SFB Online credentials and it is able to make/receive calls with other SFB users, retrieve and display the calendar for meetings information and one-touch join, search the directory and display contacts etc.. All the features that were previously on Polycom VVX phones are similar on the Poly CCX family and therefore we can use this phone to demonstrate the PSTN failover capabilities that have been introduced on the VVX phones since firmware version UCS5.8. Before we begin to configure the phone and the SBC for this failover capability, there are a few important points to note:

• ​PSTN Failover capability has only been tested on the Ribbon SBC1000 family of session border controllers. It may work on other SBCs but it will not be an officially supported scenario
• PSTN Failover relies on a secondary SIP registration to the SBC. This secondary registration remains passive during normal WAN connection and cannot be used to make or receive calls.
• The secondary SIP registration only becomes active after WAN connectivity failure whereby the phone is unable to connect with Skype for Business Server or Online
• This is not to be confused with Hybrid Line registration where both the primary and secondary SIP registrations can be active and used for calling under normal circumstances

To understand how this works in Skype for Business Online environments, the following diagram shows under normal WAN connectivity how the phones and the SFB software clients are registered to O365 SFB online:

Note that the phones have a primary active registration to Skype Online, it also has a secondary passive registration to the Ribbon SBC. During WAN outage, the Skype Online connection will be disrupted and the phones will then switch to the SBC as the primary SIP registrar to maintain PSTN calling capability as shown below:

The first step in configuring PSTN failover is to configure the Ribbon SBC registrar to accept phone registrations, if this has not yet been done. This can be done by creating a new SIP Registrar on the menu SIP > Local Registrars Table > Create SIP Local Registrar dialog box. The sample configuration in this lab is shown below:​

To configure the phone to authenticate to the Ribbon SBC, create a Local / Pass-thru Auth Table (access via Settings > SIP>
Local/Pass-thru Auth Tables). This will force the phone to provide a login and password when making a call with the Ribbon SBC.
Configure the Phone number as the User Name. The sample configuration in this lab is shown below:

Next, we go to Access Settings > SIP Signaling Group and from the top menu bar, click Create Signaling Group and select SIP Signaling Group. The sample configuration in this lab is shown below:

Finally, we create a Call Route (via Settings > Call Routing Table) from the PSTN to the phones. In PSTN failover mode, the call is routed directly to the phone using its registered IP address:

Once the SBC is configured we can proceed to load the necessary parameters on the phone. Poly provides the documentation on what parameters are required here: https://documents.polycom.com/bundle/ucs-sfb-dg-5-9-0/page/r2514147.html.  These are fairly straightforward and the sample config used for this lab is listed below:

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<!--PSTN Failover to local SBC-->
<PHONE_CONFIG>
 <
    feature.sfbPstnFailover.enabled="1"
    reg.1.server.2.address="<IP Address of SBC>"
    reg.1.server.2.pstnServerAuth.userId="+65XXXXXXXX"
    reg.1.server.2.pstnServerAuth.password="1234" />
</PHONE_CONFIG>
​
With the above parameters loaded, the phone will have primary SIP registration on Line1 to SFB Online and maintain secondary SIP registration with the SBC. During normal WAN connectivity, the phone will show the Failover State as "Inactive" as shown below:

Once WAN connectivity is disrupted, the connection to Skype for Business Online will be down and the phone will detect this and switch to the secondary SIP registration to SBC as the primary. The PSTN Failover State will show as "Active" and the phone number will be displayed:

Once in failover mode, Skype presence and features will not be available on the phone, however PSTN calls can still be made via the SBC out via the SIP Trunk. The SFB Online in-band dial plan stored in the phone is used during failover mode and locally provisioned dial plan is not used.
​
In conclusion, this blog post serves as a reference on how to configure PSTN Failover for Skype certified IP phones using a Ribbon SBC1000. Again, note the important disclaimer shown in earlier:

Important: The Poly CCX500 used in this lab is both a certified Teams phone AND a certified Skype for Business Phone. In this lab we are using it as a Skype for Business Phone to show PSTN failover capability. This PSTN failover capability is NOT supported in Teams mode at the time of this blog post.