F12 must be held to talk on "range"
range is defined as the room you are in (like tower, or hangar, or oclub) or players within a certain distance (they can be seen by bringing up your radio box)
T - is the only other voice option.
on your radio box, channel V is for "T" it can be set numerous things:
Squad
Mission
numbered channels 100-199 for people in your own country tuned the same
numbered channels 201-299 for anyone in any country tuned the same
a players name to speak directly to them (they must then tune their's to your name to talk back)
see here:
http://www.hitechcreations.com/ahhelp/radio.html