What happens if PagerDuty is down?

I’m working on designing an Enterprise-level incident response and management system for our client using PagerDuty. We’ll be using the Enterprise edition and would like to understand the implications if PagerDuty were to go down. I know that PagerDuty is a SaaS service with a 99.99% uptime guarantee. In a worst-case scenario, for instance, if AWS were to experience a downtime, would PagerDuty also be affected? I need to design an alternative solution for this scenario as well. Additionally, I’m curious about how to determine which escalation actions were taken and which were not. Anyone has some thoughts? If you have worked on similar scenario, how did you approach that?