I originally repurposed all of our Polycom phones used with another cloud SIP PBX provider. Moving them to FreePBX. If you read the Polycom docs, you can see where you can define your own provisioning server, and stick the phone config files on it. Of course you would have to host and maintain that resource to a degree. There are example config file templates for the various Polycom models. We had a mix of older SoundPoint IP and VVX desk phones, and they all worked like a champ.