Back in the day, one of the big ROI’s for Skype for Business server was bringing PSTN Audio Conferencing “in house” or “on network”. Meaning you didn’t need to pay for the external provider or for the phone bill to ring the provider from the PBX.
I did whole Skype for Business server projected that ROI’d in under a year based on just the cost of paying for external PSTN conferencing providers and the phone bill to ring them. Even if you only used Skype for Business for meetings and not full enterprise voice, we could still have your Cisco or Avaya or A Another PBX PSTN dial into the Skype for Business meeting for free via a gateway/session border controller.
Fast forward to Teams and your PSTN conferencing is provided by Microsoft, so there is a cost to pay for the provider over SfB server. It can be a flat rate per user or pay per minute. In 2020 VoIP is much more prevalent, so if you primarily use Teams you probably click to join, leading to fewer PSTN costs, but what if you are still heavily using a PBX and need to pay a phone bill to PSTN dial into those conferences.
Now, in preview, On-network Conferencing allows organizations to send inbound and outbound PSTN Audio Conferencing calls to Microsoft dial-in numbers through Direct Routing. Meaning that when the PBX user dials the PSTN Audio conferencing number it goes through the SBC on net and doesn’t incur a per-minute bill from the PSTN carrier for the call.
This capability does not allow PSTN Audio Conferencing with third-party dial-in numbers or providers. You must still use and dial the Microsoft provided numbers, you just configure the PBX to route them via a session border controller.
You can check out the documentation for how to configure this here.
Luca Vitali has also posted a good overview on his blog here.