Microsoft have stated that they would retire Exchange Online UM on 28th February 2020 and it would be replaced by Cloud Voicemail. https://docs.microsoft.com/en-us/skypeforbusiness/plan/exchange-unified-messaging-online-migration-support
This morning at 2:51am (29th Feb) we received the following email from Office 365 Message Centre.
Ensure you have configured the Cloud Voicemail prereqs. https://docs.microsoft.com/en-us/skypeforbusiness/hybrid/plan-cloud-voicemail. One of the main requirements is a SfB Hybrid which was not required for Exchange Online UM integration. It’s still not clear what has happened to customers who have not done this. Does CVM work without a Hybrid?
As per the email above you will need to migate your EXUM Auto Attendants ro Cloud Auto Attendants by end of March: https://docs.microsoft.com/en-us/skypeforbusiness/hybrid/plan-cloud-auto-attendant