At time t = 0, (N + 1) telephone calls arrive at a central office where N lines are available. Thus, one of the calls cannot be answered immediately and goes on to await the completion of any of the other N calls. The time intervals T1, ..., TN (during which each of the N lines remains on a call), are independent and identically distributed random variables with mean m exponential probability density function. Determine the mean waiting time of the call that was not immediately answered.