You have to bind the "Mic switch" keys. It's a 3 way button on the HOTAS but you can set it as your good old keyboard radio shortcuts.
Mic switch down = UHF radio.
Mic switch forward = VHF AM radio
Mic switch afterward = VHF FM radio
It corresponds to your 3 radios on the left side of the cockpit.
I just tried it anyway and my wingman never took off. He stayed still at the entrance of runway from parking just after my take off. Maybe because I checked with Chevalier before he took off ?
So atm on the last OB build it's broken ? I just finished M02 and this campaign is really great, fantastic job on immersion once again thank Baltic ! But if 03 is broken I'll wait for a new build to continue :)