Just a reminder to my post from last year that effective April 15th 2025 in the US all Voice Providers and Resellers who provide 911 calling to their customers have new regulations. This is laid out in the Second Report and Order here
You must notify impacted PSAPs no later than within 30 minutes of when the outage that potentially affects 911 service is discovered, handle all follow-up notification timeline requirements, and maintain contact information for the 911 facilities for any areas that you have customers in.
So in a real world environment if you are providing SIP Trunking and your SBC or your upstream providers SBC has an outage that keeps your customers from calling 911 you must notify every PSAP where your customers are setup with 911 registered information of the outage that keeps them from calling 911. The same is true if you are hosting their PBX and your PBX has an outage that keeps the customer from calling 911.