At the business end of the K100--the leading edge that should point directly at the driver's head--is a pair of microphones. The dual-microphone setup is there to aid the K100's DSP and echo reduction, leading to clear calls, even with the audio being piped through our car's stereo at reasonable volume. Callers never complained about being able to hear themselves or about not being able to hear us.
The third button on the K100's surface is a mute button that does exactly what you'd expect it to. Additionally, holding the mute button for 2 seconds activates a "night mode" that disables the unit's blinking connection status LED. It's a small feature, but one that we found ourselves using quite a bit during our testing.
Midway down the right edge (when viewed from the top) is a Micro-USB port for charging. Included in the box are a 30-inch Micro-USB cable and a USB car charger. Charging to full can take up to 2.5 hours, but you can expect up 17 hours of talk time or 15 days of standby. And that standby time can be stretched thanks to an auto-shutdown feature that powers down the K100 after 90 minutes of unpaired inactivity. When powered back up, the K100 will automatically re-pair with the last paired phone.
Overall, the K100's call quality is good, battery life is great, and, if you can get a clear signal for the FM transmitter, it serves a second purpose as a Bluetooth-to-FM bridge between your smartphone and your car's dumb FM stereo. However, as the issues we encountered with FM transmission are inherent in the transmission method and not the fault of the K100, we didn't factor them into our score. Even if you never plan on using the FM transmission function, the K100's loud and clear internal speaker is still good enough to handle basic hands-free calling, but only for phones that have a voice-dialing function of their own.