If you're using an existing on-premises Unified Messaging solution, moving mailboxes from the on-premises %BRAND_EXCHANGE_ONLINE_SHORT% ...

If you're using an existing on-premises Unified Messaging solution, moving mailboxes from the on-premises %BRAND_EXCHANGE_ONLINE_SHORT% mailbox server to %BRAND_OFFICE_365_SHORT% will disable Unified Messaging for the %BRAND_OFFICE_365_SHORT% users. Existing Unified Messaging services for user mailboxes that remain on-premises should not be affected by configuring a hybrid deployment. However, on-premises users will not be able to perform any Unified Messaging functions, such as transferring calls and leaving voice mail, to user mailboxes in your %BRAND_OFFICE_365_SHORT% organization. Learn more at Voice mail in %BRAND_EXCHANGE_ONLINE_SHORT% Online: Unified Messaging.