Recently had an issue with Jabra headsets leaking sound through both the Headset and Handset. I believe this was just for outgoing calls but cannot confirm.
I first thought this was a bug and found this: http://tools.cisco.com/Support/BugToolKit/search/getBugDetails.do?method=fetchBugDetails&bugId=CSCsz35375
Turns out its the default behavior of Cisco phones to do this with a headset. To Disable you need to do the following:
Router(config-telephony)#service phone handsetHeadsetMonitor 0
Restart the phones for this to work!