I understand, particularly with Office 365 anyway that you can setup a call so that if it goes unanswered and to voicemail, the call at that point is forward to Microsoft servers for the voicemail recording and subsequent integration with outlook.
Note this is more than just the VoIP provider recording the call and forwarding it to your designated email!
Some good overview documents on it here ; http://community.office365.com/en-us/w/exchange/exchange-online-unified-messaging.aspx
Does WxC (or any other provider for that matter) know about this? Will it work with any of them?