It's a rare organization that completely cuts over to SIP trunks for 100% of their telephony services. A few weeks back, Matt Brunk posted an interesting entry about his experience with an outage at ...
How would your configuration fare had your SIP trunks been down for 2.5 hours? Maybe the Internet ran out of bandwidth or someone didn't pay the light bill, but our SIP trunks were down for 2 hours ...