Using the memoryless property of exponential random variables, the fact that if , , then and , you can get the following (up to possible errors).
Suppose there are representatives, that the arrivals of clients are given by a Poisson process of parameter and that the length of a call is an exponential random variable of parameter . The queuing process is described by a Markov process on state space , corresponding to the number of simultaneously busy representatives, with , holding times at being exponential with parameter and with parameter at , and transition probabilities between "neighbours" only, given by .
If is the invariant distribution, then the average number of people on the phone per amount of time is the expectation of . This leads to the answer.