Peak busy hour is refering to a particular busy hour in which is the peak of the whole day.
Assuming PBH call is 100, you still need to know the call holding time in this specified peak busy hour.
Assuming average call holding = h in seconds,
traffic in erlang = h * 100 / 3600
Then for a given GOS, you can find the circuit requirement.
Hoping this information can be useful to you.
PS: you can contact me email@example.com for traffic related problem.