Q. How does one arrive at the probability of availability of free lines during busy hour? How can this be improved?

Ans: One can arrive at probability of free lines during busy hour by using delay probability of the exchange. Delay probability designates the delay that a caller may face before his call can be completed. In a delay system caller is made to wait until a free line is made available for the completion of call. This results in call delay. Improvement can be made if system is treated as a loss system rather than a delayed system. 

In a loss system, caller has to repeat a call till the call is established. This decreases the delay by releasing a caller from queue until there is repeated action.

Blocking probability can be useful. Blocking probability P is defined as the probability that all servers in system are busy. When all servers are busy, no further traffic can be carried by system and arriving subscriber's traffic is blocked. At the first time, it may appear that blocking probability is the same measure as  GOS. Probability that all servers are busy may well represent the fraction of calls lost, which is what GOS is all about.