I don't know. One of the most frequent problem I see is power outages at
the sites. I don't think any ISP can be responsible for things like that.
Let's assume that you measure "availability" as being to ping the
customer site router from some site off your network. Some ISPs install a
UPS on the router at the customer site, some don't. That decision effects
price and reliability and "availability" (especially when the customer
has a UPS on his own equipment).
Or let's look at lightning strikes. Some ISP's say "that's an act of
God, we can't be responsible". Others put in surge supressors, have
hot swap backups, etc. and they keep running.
Some ISPs say "that's the fault of my upstream provider, we can't
be responsible". Others tell their upstream provider to do a better
job or they switch providers.
Much of it sounds like a cop out to me. Ultimately, (with enough money)
there are few reliability factors in this business that are truly out of
anyone's control. It's just very convenient to say that there are.