One can arrive at the probability of free lines during busy hour by using the delay probability of the exchange. The delay probability indicates the delay that a caller may face before his call can be completed. In a delay system the caller is made to wait till a free line is made available for the completion of the call. This leads to call delay. Improvement can be made if the system is treated as a loss system instead of a delayed system. In a loss system, the caller has to repeat a call till the call is established. This reduces the delay by releasing a caller from the queue until there is repeated action. Blocking probability can be useful. The blocking probability P is defined as the probability that all the servers in system are busy. When all the servers are busy, no further traffic can be carried by the system and the arriving subscriber’s traffic is blocked. At the first instance, it may appear that the blocking probability is the same measure as the GOS. The probability that all the servers are busy may well represent the fraction of the calls lost, which is what the GOS is all about.