Hey,
the functions are based on user licenses not server or application based. But you don't need to install and configure the other applications. But check out the feature table of the licenses. A lot of stuff is always included and you could use it.
You will find details about the licensing here: http://www.cisco.com/c/en/us/products/collateral/unified-communications/unified-communications-licensing/product_solution_overview0900aecd806cc7a4.html
Be aware that you need for some phone types a minimum license(table 2).
You could install the subscriber on the same server as the publisher.But yeah, that is only a software redundancy. So for a real physical redundancy you will need a second server, but there is no additional license necessary.
chris
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