Hello rmac,
welcome to the Polycom Community.
Polycom, just like any other SIP endpoint manufacturer, simply provides phones that can be used with a vast range on SIP services.
Therefore we are unable to determine who you are using your service with. It could be a well known service provider or using some homemade free PABX like Asterisk.
You could check via Menu > Settings > Status > Lines if there is a human readable information there as this may give you a clue who your service provider would be,
If the above fails you need to speak who set this up.
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