Expandable, digital answering system makes keeping in touch with family and friends easier with advanced answering machine, advanced features and four cordless handsets. The KX-TG684 is expandable up to six handsets and also comes equipped with noise reduction so you can hear the person calling you. Power backup operation lets you make and receive phone calls with the cordless handsets during a power outage. Key Detector-compatibility lets you attach an optional key detector to a frequently misplaced item and use your cordless handset to locate the lost item. Talking Caller ID feature uses text-to-speech technology, announcing the caller's information in between rings. Intelligent eco mode automatically lowers the power consumption when using the handset close to the base unit to extend the available talk time. DECT 6.0 Plus technology delivers superb sound quality and outstanding range. Engineered to work in 1.9GHz frequency, the phone won't conflict with common wireless devices.