The radio communications are basic. The pit will radio you when you have done one or several of the following:
a personal best lap time
the fastest lap, either in the race or qualifying
you set the fastest lap in qualifying and gain P1
when you have lost the fastest lap
when you are on the last lap of a race
These are the messages I hear when I play GP3 1.13 and I think they may be the same in GP3 2K.