SfB Online - Failover to on-prem PSTN in case of internet connectivity issues.

Hello,

We have a SfB Online deployment with Microsoft managing the PSTN/numbers. Now we have a requirement where some sites with a single internet connection and link to internet goes down, they should be able make outbound calls/911 calls. we have Trio 8800 and VVX Phones, we can install analog lines and order phones that supports SIP line as well as backup Analog. 

Has anyone done similar setup and what kind of configuration and hardware is required? 

Thanks