It has been discovered that the latest Apple iOS upgrade 7.1 for iPhone, iTouch, and iPad devices breaks current Lync 2013 functionality. Specifically, it has been found that the update creates interoperability issues which causes voice and video to fail when using these devices. Microsoft is aware of the issue as seen in KB2955745 but there is currently no fix or workaround available.
The current solution? Hold off upgrading your Apple devices until a fix is found or you may be without Lync Audio/Video (I say might as Apple iPad Air and iPad Mini do not appear to exhibit the issue).
An update has been released and availabile in the iTunes store and may be foundhere. This update addresses the Apple flaw.