T41S Common Area Phone not answering

[SpencerportCSD]

Are purchased Yealink Y41S phones for some common area phones in Skype for Business 2015 on premise. We login using Pin authentication. We can make calls out from the phone, however when we try to call the phone we get a message "is in another call". Traces from the phone show the call coming in but the phone itself is saying it is Busy.

Any ideas of what to look at.