A Graph Theory Problem

Suppose there are N people. They are trying to make a way to have a communication system such that all N gets the information one gets, spending least time and cost. Assume that the only way of communication is via telephone. Assume 1 person can make only n (<N) phone calls a day. Each phone call takes exactly t minutes (N  t < 1 day). What is the optimal time taken for all N to know the information?