Does anybody know of any study that lists the most likely modes of failure for leased lines?  Specifically need info on what the most likely modes of failure are for my T-1 lines so I can plan a fault tolerant scheme based on failure probabilities.

I'm not aware of such a study but leased lines are an very mature technology and typically network designers build in the appropriate resilience to protect against the usual failures.

I've implemented a fair number of leased line installations and without a doubt the most common fault for me is a fault within the Telco cloud.

Therefore I typically find the most resilient solution tends to be a dual router/dual telco solution. For ultimate resilience, you may need a seperate duct for the 2nd leased line feed (can be enormously expensive if that duct doesn't exist) and ensure the routers and NTE's are protected by a UPS with seperate power feeds.

I was hoping for some numbers so I can assign probabilities.  I have 3 offices with 2 T-1's into each.  Our office in San Francisco went off line for 4 days because a man hole flooded during the recent rains.  For now, fault tolerance is a hot topic with the bosses and I need something to present them that includes numbers.