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
- 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:
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:
<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<!--PSTN Failover to local SBC-->
reg.1.server.2.address="<IP Address of SBC>"
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:
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.