<p>One of the important policies of governments in different countries is the expansion of the mobile phone market in order to make the market more competitive with different services and reduce prices. In this paper, the impact of a newcomer operator on the Iranian mobile phone market with three existing operators of the Mobile Telecommunication Company of Iran (MCI), Irancell, and RighTel is investigated. The three existing operators are assumed to be in a three-way Cournot competition (CC) before the entrance of a newcomer operator in which each operator decides on its output and price independently. By entering the newcomer operator to the market, a four-way CC (i.e., no coordination) is formed and the price and profit of each operator are obtained. Furthermore, moving from the four-way CC to monopoly cartel (i.e., full coordination), partial cartel and Stackelberg competition (i.e., partial coordination) is studied. The results show that increasing the intensity of competition in four-way CC is in the interest of total industry profit. Furthermore, moving from the four-way CC to other competition models is not recommended due to the high market share of MCI.</p>