A portable phone is equipped with a position detector that detects the geographical position of the portable phone, and a telephone system may ring one or more land-based phones or portable phones according to the geographical position of the portable phone. One or more geographical regions are defined, with corresponding telephone parameters defined for each region. When a portable phone enters a defined region, exits a defined region, or changes between two defined regions, the function of one or more land-based phones or portable phones may change. The preferred embodiments thus allow changing the function of one or more other phones based on the self-detected geographical position of a portable phone. In this manner the convenience of using portable phones is greatly enhanced.