The transformer coupled stereo output provides the local mic on the left channel and the caller on the right channel. Local audio is a full bandwidth connection, meaning your voice will sound considerably better than the caller. Of course, audio quality is limited by the quality and placement of the microphone on the ear-bud style headset.
CellTap 4C™ will not interfere with headset button controls on most devices. A headset is required for operation, as the phone will not recognize CellTap 4C without a headset connected.