Grandstream HA100 High Availability Controller for UCM6510, a failover solution for the UCM6510 IP PBX. Connect two UCM6510s to the HA100 to create a back-up solution that automatically switches to a secondary UCM6510 (slave) if the main UCM6510 (master) fails

http://www.grandstream.com/sites/default/files/Resources/HA100_english_datasheet.pdf