The 2.4 GHz signal ensures stable and smooth transmission. The effective transmission distance can be up to 300 meters. Two-way intercom: When someone presses the call button of the outdoor unit, the indoor bell will ring immediately to remind you. You can press the answer button to speak with people outside. Built-in high sensitivity microphone and speaker, enable audio input and output, ensure clear and smooth sound effect in communication. Ultra-long standby: the inner bell is powered by rechargeable lithium battery, the outer bell can work for more than 6 months with 4 x AAA batteries, energy saving. Simple and Stylish: Ultra mini size ensures perfect portability and easy installation, small and nice, easy to install.