Poisson Distribution in Telephone System

Hi! Can someone help me with the following problem:

Suppose that there are `m` lines in telephone system. Each line is used to connect a pair of disctinct subscribers. Furthermore, subscribers call at an average rate `L` according to a Poisson process and only call those that are not currently involved in a call. In other words, the number of calls

attempted in a given time interval has a Poisson distribution with parameter “L”.
Calls made when there are currently m calls already in the progress are lost, and the
duration of a call has an exponential distribution with mean 1/meu. What is the throughput of this system?