The paper investigates the possibility for competing network operators to cooperate partially without operational information exchange and to share a block of UMTS carriers simultaneously to provide services. The performance of spectrum sharing in the worst case for sharing (equal capacity demands) is investigated for the downlink of the UMTS FDD system. This work continues from earlier work (Pereirasamy, M.K. et al., Proc. 5th World Wireless Congress, 2004; Proc. 15th IEEE Int. Symp. On Personal Indoor and Mobile Radio Commun. - PIMRC, 2004), extending the initial concept to take into account that operators would realistically deploy operating networks composed of separate network equipment and infrastructure. The possible gain that is offered through spectrum sharing compared to conventional systems, with only proprietary carriers for operators, is studied for speech type traffic. It is found that the relative displacement of cooperating networks has a negative impact on the trunking gain given by the spectrum sharing mechanism. Considering only speech service type, a maximum allowed network displacement is given for a spectrum shared environment with the cellular network parameters chosen for this investigation.