When I made my RingCentral account I already owned several Polycom phones; a few IP335s purchased from our previous VoIP provider, and some IP550s from when I ran my own Asterisk phone server. So I didn't want to purchase any more phones. Fortunately there is good documentation online and I had no trouble at all. It only took a few minutes and I had all of the phones working perfectly.
Here are a couple of links to pages that might be helpful: