Of the numerous performance metrics applied to cellular telephone systems, probably none is more important for customer satisfaction than the system drop call rate. Customers are more sensitive to call dropping than to call blocking at initiation. Proper system design and operation involve keeping the drop call rate as low as possible. Call/packet dropping refers to the event described as the termination of calls in progress before either involved party intentionally ends the call. There are numerous drop call causes in cellular networks with majority of them occurring in the Um interfaces mainly due to lack of radio resources created by electromagnetic causes and user mobility (ie handover). Another important contributor of drop call rate is the traffic load in which, the call arrival rate and holding time play significant roles. Drop call probability is defined as the probability that a call is terminated due to one or all of the above-mentioned causes and is basically estimated from drop call rate by applying the Poisson probability distribution function. Dropcall probability has been the subject of several network performance studies and a major contributor to service optimization in established cellular networks. In this paper, we present an overview of drop-call probability factors in cellular networks. Moreover, some of the factors have been analyzed to study the trends in relation to an operative GSM network and the results are discussed.