Truth be told, I've haven't run into that problem nor is it a known issue. But hey, I can imagine how this would be a useful utility if you take conference calls often so let's work on getting it fixed.
First of all, I have to ask how long you've been having this issue? While we wait to hear back, we've got some troubleshooting to take you through. First, go to settings > application manager > all > phone > clear data. Next, go to settings > about phone > software update > check new (so as to make sure you're running the most up to date software. Finally, turn off your phone, turn it back on, and test. Please share results so we can plot a new course of action if necessary.
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