The Omnivoice service cannot be enabled for a user: you either don't see this offering item or cannot enable it.
The service may not be available in the region/DC. The service is currently available in the US.
If you don't see the Omnivoice offering item among available items, this offering item is not enabled on tenant level. Do the following: see Enabling or disabling offering items.
If this offering item is available but you cannot enable it, contact Acronis support for assistance.