Hello Joe,
again I can only hghlight the existence of the Community FAQ.
The community's VoIP FAQ contains this post here:
Nov 4, 2011 Question: How can I setup a Busy Lamp Field / BLF with an Digium Asterisk SIP Server?
Resolution: Please check => here <=
Please ensure you always check the community FAQ and/or utilize the community search before posting any new topics or follow up post’s.
The above shows various examples. These should enable you to utilize Wireshark to check the signalling or utilize the phone logs.
The VVX500 in this case was sold via Siphon Networks Ltd back in 12/11/2015 so contacting them for Tier 1 support would be the next step if the above FAQ does not help you.
Please ensure to provide some feedback if this reply has helped you so other users can profit from your experience.
Best Regards
Steffen Baier
Polycom Global Services