Squad channel (red) can be used for voice as well, but heard/seen only by members of the squad.
numbers chan can be used by whomever is on same channel.
I dont like the idea of sqdvoice invite for the numbers channels, for one simple reason: Who decides wich squad gets wich channel and why? what about the new squads? how many members in a squad until they get their own chan?? i think it would only cause trouble.
Besides, Im not currently in a squad, but sometimes when winging with a friend, we just find ourselves a chan thats unoccupied. If we happen to be on someones squad chan, we leave.
Missions: they usually use some mission (number) chan. Would be a bit unpractical to have to invite every mission member at start and throw them back out at the end of mission.