How Can I solve this problem?

Problem 1:

Telephone calls arrive at a department store catalog ordering center with two operators. If both operators are busy, the calls are automatically put on hold and queued on a first-come first-served basis to be serviced by the next available operator. However, there is a limit to the queue space. A maximum of 2 calls can be put on hold and queued. The time spent by each operator in taking the order is exponentially distributed with a mean service time of 6 minutes. The calls have exponentially distributed interarrival times and occur at a mean rate of 15 per hour. The following four questions are for hand calculation with the rate diagram.

1) Construct the rate diagram for this system;

2) What is the expected number of calls on hold waiting for service?

3) How much time should an arriving call expect to spend in the queue?

4) What fraction of time are both operators idle?