Hi Max,
I hate to be the bearer of bad news here but....
The 7815 will not work here When installing Unity Connection there is a very
stringent hardware check that will not pass with the 7815;
Cisco Unity Connection 7.1 Supported Platforms List
http://www.cisco.com/en/US/prod/collateral/voicesw/ps6789/ps5745/ps6509/data_sheet_c78-491743.html
The recommendation for Unity Connection clusters is that both servers are identical or
at least equal in their capabilities. You would require a HA (High Availability) license as
well as a license for the subscriber and new subscriber VM port licenses as well.
Cheers!
Rob