Thanks for posting. You will likely want to work with your provider to make any changes to call routing in the unit. However, your unit will support survivability, and below is a link to a guide you may find helpful. This shows the configuration of an FXO port to be used full time for 911 calls, and a backup PSTN path when the SIP server is not reachable.
Feel free to add any follow up questions you may have or additional details about your current application and the backup paths trunks you wish to incorporate.