Call twinning is where a call rings on a primary device before the call is 'twinned' to other devices on the User object. This is useful when multiple devices and or external numbers are configured on a single User object, and one of these, such as a desk phone, needs to consistently ring first.
Setting up call twinning is configured entirely on the User object. To enable it, the ring mode on the User must be set to ring all. There will be a check box for 'Enable device twinning mode'. Once enabled, the first device on the User will ring for the time specified on the slider, before ringing all other configured devices simultaneously, the the time in seconds configured in the twinning mode window.
You can change which device, Microsoft Office 365 account, or external number is the primary device by dragging the desired device to the top of the list.
Don't forget to Apply the Configuration when any changes are made to the user or associated objects.