apart from the fact that the Software on this VVX1500 is rather old I cannot confirm that a second call is not being displayed.
A regular SIP registration when a second call arrives on the phone shows:
After a timeout this menu minimizes itself to:
Please ensure that you check the User Guides => here <= and also in addition the community FAQ.
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