the answer is still pretty much the same as I already provided in your post => here <=
If you believe the phone is not behaving SIP RFC conform then I suggest you work with your Polycom Reseller and / or Polycom support.
Notice: This community forum is not an official Poly support resource, thus responses from Poly employees, partners, and customers alike are best-effort in attempts to share learned knowledge. If you need immediate and/or official assistance please open a service ticket through your proper support channels. Please also ensure you always check the VoIP , Video Endpoint , Skype for Business , PSTN or RPM FAQ's