An update on this issue...
It's still going on. The user had two instances of this today and it's only 10:45am. I just experienced it msyself also. The user called me as normal, we spoke, then hung up. I walked to his office, had a chat, then left. He then calls someone else entirely and chats to them. Upon hanging up that call, his phone immediately dialled mine. Back at my desk at this point, I answered and he said he didn't initiate the call.
So it's certainly not the caller requesting a call back. Any other ideas?
Thanks