My guess is that you have tuned radio 5 to yourself and left it there.
Change radio 5 using the mini menu to "mission" or "squad".
If you are flying with people that are all on the same channel then tune radio 5 to that channel and use T to key the mic. You will transmit to all on that channel.
F12 is another mic key. It doesn't use a selected channel, it just transmits to all within icon range. When you are in a local fight and wish to transmit to others in your area use F12.
Also. The Tower and outside the tower are seperate 'rooms'. If you are in the Tower you can only use F12 to chat with those also in that tower. The opposite is also true.
http://www.flyaceshigh.com/ahhelp/radio.html