Features:  1. The button supports up to 20PCS, and the receiver supports up to 100pcs.  2. If multiple buttons are paired with one or more receivers in the same system, each button can be set to a different ringtone.(This feature supports the sos button.).  3. Power-off memory function: Turn the power on again after power off, no need to re-adjust and lock the ringtone.(This feature supports the sos button.).  4. The doorbell transmitter and sos button has no screw chuck design, and it can be waterproof and dustproof without rubber ring. 5. The transmitter USE a 23A12V alkaline battery (not included battery).  6. The sos button USE a CR2032 battery.(not included battery).  7. Receiver voltage: 110-220-240V.  8. The receiver is available in a variety of plugs, US/EU/UK Plug and AU adapters, suitable for most countries.  9. Doorbell Receiving distance:300 meters in open area.  10. SOS button Receiving distance:80 meters in open area.  11. The wireless signal will be affected by obstacles. The more obstacles, the more serious the signal loss.  12. Receiver has 5 levels(can be mute) 0-110db,60 music to choose from.  13. Size: Transmitter:75x51x20mm, SOS button: 60x34x11.5mm, Receiver:95x65x24mm.     Package Includes:  1 x Transmitter.  1 x SOS button.  1 x Receiver