A cordless telephone instrument is improved in that, even when a portable unit is not in use, it can be determined and notified to the user whether the portable unit is inside or outside of an operational area of the portable unit. In portable unit 2, portable unit timer 28 counts a predetermined first...http://www.google.de/patents/US5644620?utm_source=gb-gplus-sharePatent US5644620 - Method of indicating a portable unit of a cordless telephone instrument out of operational area and its device
