When sending somebody e-mail you assume they'll probably check their e-mail and receive the message eventually, but you have no idea if they'll get it right away, or if they'll notice it along with all the other e-mail
Thus spake "Steve Gibbard" <scg@gibbard.org> they get.
When phoning somebody, you know right away whether they answer, and you know right away how they respond to whatever you have to say.
In theory, that's a job for DSN.
If you really need to get in touch with somebody right now, do you call their presumably more reliable land line, or their presumably less reliable cell phone?
I think this was intended as rhetorical, but I'll bite... If I want to reach someone _right now_, I call their cell phone, because the odds are significantly higher they'll be near it. E911 is a mess even for non-mobile users in a single office building. Full compliance, at least with my state's regulations, requires phone numbers be localized to within a 100ft2 space. This means a VoIP system must be able to automatically trace a user's IP address, MAC address, physical port, cable run, and at least portion of a floor -- not to mention figuring out what the correct telco circuit to route the 911 call out, which obviously varies depending on where the user happens to be sitting. S Stephen Sprunk "Stupid people surround themselves with smart CCIE #3723 people. Smart people surround themselves with K5SSS smart people who disagree with them." --Aaron Sorkin