4.5. A ticket office has two agents answering incoming phone calls. In addition, a third caller can be put on HOLD until one of the agents becomes available. If all three phone lines (both agent lines plus the hold line) are busy, a potential caller gets a busy signal, and is assumed lost.

Suppose that the calls and attempted calls occur according to a Poisson process of rate A, and that the length of a telephone conversation is exponentially distributed with parameter μ. Determine the stationary distribution for the process.