3.1) Can I fly Space A on flights from other services?

Yes.  Most people think of the Air Force, and in particular the Air
Mobility Command (AMC) when they think of Space A.  Actually, Space A
passengers are eligible to travel on any suitable DoD-owned or
controlled aircraft and on Coast Guard aircraft as well.  Because
passenger movement is part of their mission, the Air Force has by far
the best system of passenger terminals which provide flight
information and process passengers.  But even at bases with Air Force
passenger terminals, those terminals may not always know about or
handle other services' flights.  The smart Space A traveler will seek
out information about every activity that provides flight information
and processes passengers at every base near their origin and
destination and then call them directly in order to get the best
possible picture of what is available.

3.2) When should I call for flight information? 

If you've never traveled on a particular route before, you might want
to call the passenger terminal you plan on traveling through a month
or so before you plan to travel.  At that point they will be able to
discuss their typical flight schedules, Space A backlog, any movement
forecast they know of for your desired travel period, and how far in
advance they will know their schedule.  This is also a good time to
sign up for a flight, which you can often do by fax.  Then call again
a day or two before you are ready to travel to see how the schedule is
shaping up.  Most passenger terminals will have a schedule for the
next day's flights, and some will know further in advance.  But these
schedules are subject to change, and often they change a lot.  So once
you're ready to travel, you should stay in close touch with the
terminal in case something comes up on short notice.

3.3) Where do I get the phone numbers?

The phone numbers can be found in guidebooks, on information sheets
provided by the passenger terminals, and through the Space A World
Wide Web home page on the Internet (see question 6.3 for the address).
Several passenger terminals now have their own World Wide Web pages,
and many of them also list phone numbers.