When the magnetic switch is activated (doors or windows), the telephone transmitter (sold separately) is activated and the user can be alerted through any of the Bellman Visit receivers. 

Features 

• Must be used with the Visit Telephone Transmitter (SKU 5106)
• Activates the telephone transmitter which then alerts you through any of the Bellman Visit receivers 

Specs

• Cord length: 1.6ft
• Connector: 3.5mm Mono jack plug
• Weight: 1oz
• Open Switch: <1cm from magnet
• Closed Switch: >2cm from magnet
• Range: 100ft 

Documentation