I had this problem and could not find a clear answer. The Polycom IP 6000 conference phones are now available for under $50 refurbished rather than 10 time that price new. That means a lot of people are going to buy an inexpensive Conference phone. The problem is that the phone can be configured to work with RingCentral, but they go into a BOOT LOOP causing them to re-boot every 30 seconds or so. The problem is that the latest firmware, which is installed on the phone, is not compatible with RingCentral. Here is the fix.
1) Plug the phone in and get connected to your network.
2) find the ip address (Menu, Status Ip)
3) go to the phone in a browser. The default password is 456
4) go to Utilities/ Software Upgrade/Check for Upgrades.
5) click the down arrow and select 126.96.36.1991
6) click Install
7) say yes to warning that the phone will be downgraded, then accept , then OK
8) Phone will reboot after firmware install.
9) Provision as normal using the MAC address