It depends what you have now. If you have IPBase and want to upgrade to IP Voice you need the license FL38-IPB-IPV= as outlined in the Cisco 3800 Q&A.
The question if you are allowed to upgrade from 12.3 to 15 depends on your support-contract. And if you come from 12.3 it's very likely that you also need a RAM and Flash-Upgrade.
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