Please note you can only use these phones if local to 3CX server - do not support SBC or STUN. If using phones remotely, you would have to setup a site to site vpn
See limitations - https://www.3cx.com/sip-phones/polycom-vvx/
Founded in 2005, when VoIP was an emerging technology, 3CX has gone on to establish itself as a global leader in business communications.