Say there is an outage at the 911 call center. Now you try to call, don't get through, and your phone writes off that tower. Who were you planning to call after 911? Too bad, should have placed that call first.
Your phone would try other towers from other providers. If 911 is experiencing an outage that’s a separate issue that needs to be mitigated at a different layer. Even still, 100% uptime is difficult and expensive.
