Keep having problems with legacy AD data in user objects from on old LCS install, and then keep forgetting where I’ve found what to clear out from before.
Basically, set these three:
msRTCSIP-FederationEnabled = True
msRTCSIP-PrimaryUserAddress = sip: firstname.lastname@example.org
msRTCSIP-UserEnabled = True
to <not set>. The user should then show up when you try to enable a user in Lync 2013.