The OhMiBod runs off audio signals, but it doesn't require both audio channels to operate so I send the audio signal from the app through one channel (i.e. the right speaker) while the phone conversation runs through both. To have a conversation that doesn't include the clicking sound I generate to pulse the vibrator, you just listen in on the left headphone.
Now, that means the conversation is also driving the vibration, but that can be fun too. :)