The VVX300 is not an officially supported Phone for the Cisco CUCM platform.
Any interoperability testing as described in detail => here <= has only been archived for the SoundStation IP 7000.
The above mentions as a limitation:
Music on Hold (MoH)
Supported Polycom phones can receive MoH when placed on hold by another Polycom phone, but CUCM does not support streaming MoH to Cisco phones when the Polycom phone is put on hold.
I would suggest to raise this with Cisco support in order for them to add MoH Support for 3rd party SIP devices.
Please ensure to provide some feedback if this reply has helped you so other users can profit from your experience.
Polycom Global Services
---------------- The title Polycom Employee & Community Manager is a community setting and does not reflect my role. I am just a simple volunteer in the community like everybody else. My official "day" Job is 3rd Level support at Poly but I am unable to provide official support via the community.
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