We had an interesting issue a while back where a user was able to use the Lync (Skype for Business) client for IM, desktop sharing etc. The problem was the user could not make/receive any peer to peer or Enterprise Voice calls. The call would try and connect and then cause the Lync/Skype for Business client to hang. A malware scan was run after some troubleshooting and malware was found. The user was able to make and receive calls after we got rid of the malware