Hello miarec,
welcome to the Polycom Community.
The functionality of the Polysoftkey Interface is reserved for the VVX1500 and the formerly owned SpectraLink 84xx series of phones.
The newer VVX models no longer support this and we should update the SDK.
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