We have Skype for Business hybrid in place and Exchange Hybrid in place.
Skype for Business 2015
We have setup Skype for Business online oauth with Exchange On-premise setup, so our SfB online users can record voicemail greetings.
We have found only our SfB online users configured with our on-premise primary email domain can record their voicemail greeting out other users with different domains cannot as the voicemail change greetings button in the sfb client is greyed.
We have tried moving them from Islands mode to Teams only and when they record their voicemail greeting in Teams it does not save the voicemail.
Their mailbox is on-premise, but as part of testing I have moved their mailbox online and it records fine. We do not want to migrate users mailboxes to Exchange Online yet, so need to get this working for these none-primary domains whilst their mailbox is on-premise, but their sfb account is online.