NOTE: We are hearing feedback from users that if you have Lync Server 2013 with Live Messenger PIC enabled and the Skype user has Skype for PC Version 6.2 audio calls should work.
On November 19, 2011 it was noted (see blog/how-to) that Skype to Lync federation seemed to start working at the IM & presence level after users updated to latest version of the Skype application and merged their Live Messenger ID into Skype using the latest Skype client.
At Lync Conference it was announced that indeed, merging LiveMessenger/Microsoft Account and a new version of Skype will be the actions that enable Lync <-> Skype federation. (blogger Greig Sheridan has done a good job of listing the requirements to get Lync <-> Skype federation working here: Click Here) Also noted was that Lync <-> Skype federation IM, Presence & Voice would be here around June 2013 ETA/timeframe. It appears that while no announcements have been made, Lync <-> Skype Voice federation has started to work for some and at various levels for others. See @shawnharry note below.
@voipnorm has it working both ways with Lync 2013
@MichaelVerso Notes it works one-way for him:
At the moment my experience has been that I can make one audio call from Lync (Skype PC version 6.1.x) –> Skype (Skype Android 184.108.40.20658) and audio flows, but after hanging up I can’t make another call. Also, I’m noticing that the incoming call notification to Skype indicates “unknown caller”. It is interesting to note that in the Skype call history the call is listed as “undisclosed_pstn:xxx”. I don’t think this necessarily means Skype is using pstn to deliver the call: it may just be a generic message if caller ID is note received correctly?
So, to summarize: Microsoft has not announced Lync <-> Skype voice federation to be ready at this time, so don’t expect it to work. But it does look like it is working for some, so if it does for you: Enjoy. Also, it appears Microsoft is making progress towards their mid-year ETA release goal.