In a standard handset, the speaker is physically isolated from the microphone. In a hands-free system, the far-end speaker's voice comes out of the loudspeaker, is picked up by the microphone, and is sent back to the far-end user, creating a distracting echo (often perceived as feedback or talking into a "hallway").

One of the hardest problems in hands-free calls is "double-talk" (when both people speak at once). The system must not cut off the near-end speaker while canceling the echo.

If you are conducting a literature review, these are the key categories and specific papers that define the field. The core challenge in hands-free telephony is and Noise Reduction .