Here at BVoIP we support a full list of Polycom Devices including the Polycom VVX phones with Side Cars. It's worth mentioning that the Polyxom VVX Side Cars work a bit differently than most other manufacturers....
In most cases, BLF keys show up on side cars whereas if an extension is not on a call the button light is green and when they are on a call it is red.
In Polycom's case if the extension is not on call the button shows NO COLOR then when there are on a call the buton is RED.