When useing Lync client on Iphone one of my coworkers has seen this issue. When someone tried to call him the call just went directly to his Voicemail.
After different testing and working on this, well it seems like the issue was the Lync client on the Iphone was the issue. The error message we got was something like "can not log you in right now"
We then did an unistall of it and then we tried to call him again.
And now the phone was working just fine.
Then we reinstalled the Lync client for Iphone again and it all worked smooth.