Our parent company, Dynamic Net, Inc., uses the monitoring system we developed primarily for NTT/Verio's enterprise division. It includes SLA reporting, and has extremely low false positives (two false positives in four years of operation).
You could also use other monitoring systems as well such as Alertra.
From time to time we get up time requests; we run them reports out of our system.
Since they are able to verify NTT/Verio's enterprise division (where typical customers paid $50,000 per month for managed hosting) used our monitoring system, they are ok with the reports we give them.
We do have one client where up time matters greatly, and they use our monitoring system plus two others.