With no danger in sight, males make three call sequences. The first - a pair of booms - is made when the monkey is far away from the group and can't see them. It's a summons that draws the rest of the group towards him. Adding a krak-oo to the end of the boom pair changes its meaning. Rather than "Come here", the signal now means "Watch out for that branch". Whenever the males cried "Boom-boom-krak-oo", other monkeys knew that there were falling trees or branches around (or fighting monkeys overhead that could easily lead to falling vegetation).
Interspersing the booms and krak-oos with some hok-oos changes the meaning yet again. This call means "Prepare for battle", and it's used when rival groups or strange males have showed up. In line with this translation, the hok-oo calls are used far more often towards the edge of the monkeys' territories than they are in the centre. The most important thing about this is that hok-oo is essentially meaningless. The monkeys never say it in isolation - they only use it to change the meaning of another call.
As complex as their language is, Campbell's monkeys can only communicate things that they see or experience in the present. Link