The UniFi Controller by Ubiquiti Networks, Inc. is required for UniFi phones to work with the OnSIP Hosted PBX. You will setup each phone individually, but you will use the UniFi Controller to adopt the phone and upgrade the phone.
All of the PBX functionality like Attendant Menus, Voicemail boxes, and call flow will be configured on the OnSIP Hosted PBX Admin interface.
Unbox your UniFi phone and plug it into a PoE connection on your network.
To configure the UniFi phones, first you must adopt them and then upgrade them via the UniFi Controller. Start the Controller on a computer on the same network. Choose "Devices" on the left.
First choose "Adopt" next to the new phone. Then choose "Upgrade". In order for the phone to work on the OnSIP Hosted PBX, it needs to be running at least 4.6.2.
Once the phone is adopted and upgraded, you are done with the UniFi Controller. The rest of the settings are done on the phone itself.
Thaks Nate. The above article has been heavily edited to reflect the update.