Hi, Nana!
I think that interoffice means your subscribers will make only 10% of their calls out of the exchange(switch) you are designing. That is equal to:
1000(subscribers)x0.02(Erl)x10%=2 Erl. Only 2Erl will be yor outgoing traffic, internall traffic will be 18Erl. The other case is just the oposite.