Queuing theory has been proved as an essential and important tool for performance evaluation of telecommunication networks. In this way, Erlang-B formula has been used frequently for capacity planning of circuit switched networks. Consequently studies of capacity planning in mobile cellular networks are also based on such a formula. In this study we show that the voice traffic model in existing cellular networks does not follow negative exponential assumptions and therefore the Erlang-B formula is no longer valid. Our study considers four cells in different geographical places in Isfahan and studies the traffic behavior of voice services. Our study shows that regardless of geographical situation of the cells, the interarrivl time and call holding time of voice calls are identically independent distribution (iid) random variables which are not negative exponential. The blocking probability of voice traffic produced from our new model has noticeable difference with those obtained from Erlang-B formula.