We're 100% cloud (Office 365 E3) with Cloud PBX / Skype for Business Online;
The Exchange admin panel has an option to enable Unified Messaging for our users -- is there any benefit to doing this -- and what would the dial plan be?
Not something I have dealt with but it seems to enable voice mail features:
Voice mail in Exchange Online: Unified Messaging
Check Skype for Business voicemail and options