This week on Microsoft Ignite 2020, Microsoft announced Teams Survivable Branch Appliance with Teams offline Mode.
With this option, it is possible to make voice calls between Teams clients and PSTN calls through the local SBC if internet connection is not available.
At the moment I have no technical deep information, but you have to install some Teams software on a local server to support the Teams offline mode.
If I get more technical deep information, I will post on my Blog.
This picture is from the Ignite 2020 Session
The Audiocodes SBC support now PSTN calls for IP Phones, Analog Phones and the Teams desktop client without connection to the Microsoft Datacenter.