Hi,
it seems that right now it's not possible to use the radio mode of the voice chat feature while using a JTAC/GCI slot (in CombinedArms). Ctrl+Shift+Tab shows the window in room mode, but you can't access the radio mode.
In the voice chat settings, there's no unit for JTAC/GCI so no way to even "add" a custom radio or something.
So you can't actually talk to the pilots in the air.