Through this feature, you can have multiple mobile devices (such as an iPhone and an iPad) assigned to the same user on your account.

To do this you just need to go to the app you have assigned to your user and go to the settings > My Devices and click ‘Link a  Device. Doing this will open up a QR code.

Then on the secondary device, you just need to download the app and upon opening it click ‘Scan QR Code’ which will bring up a QR scanner with which you just need to scan the QR that you have on the first device.

Once you have scanned this QR code the secondary device will be set up against the same user.

Do note that once you have both of the apps activated that they will work in tandem so that if one rings the other will ring at the same time as all devices assigned to the same user will act as if they are on the same line.