Mobile telecommunications operators offer telephone services when the subscribers roam to other countries. However, a call setup to a roaming mobile subscriber may result in two expensive international trunk connections. In this article we propose a plug-in solution to avoid the international trunk connections. By adding public switched telephone network gateways, our solution does not need to modify the existing mobile telecommunications systems. In this solution a timer is defined to reduce the call drop probability. A prediction method is proposed to determine the timeout period such that the call drop probability is limited within an acceptable range. Numerical experiments are conducted to indicate the performance of the prediction method. Finally, a prototype implemented at National Chiao Tung University is described.
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