Hi,
The IP Telephony Class of Service defines what numbers are allowed to be dialed or received by phones that are configured to use that Class of Service.
The IP Messaging Class of Service defines common parameters among mailboxes that can be applied to mailboxes that are members of that Class of Service. The mailboxes in a Class of Service can also be Initialized, or reset to factory defaults. This is a feature commonly used by school systems during Summer break.
The IP Telephony Type of Service defines the features that are allowed to be used by phones and the common Button Mappings that can be assigned to phones that are configured to use that Type of Service.
I hope this helps to explain the differences and similarities between them.