In terms of form factor, the Plantronics K100 is a fairly basic visor-mounted speakerphone. Its rectangular shape and low profile are reminiscent of many similar models that we've tested, such as the Parrot Minikit Slim and the BlueAnt S4. Measuring about 2.25 inches wide by 4.5 inches long by 0.5 inches thick, the K100 is about the same size, as well. Its top surface is home to a trio of silver buttons and a rotating dial that controls the volume of the unit's 1.25-inch loudspeaker, while the bottom panel is where you'll find the metal spring clip that holds the unit in place on a vehicle's sun visor.
Pairing and calling
Inset in the center of the volume knob is a multifunction button that serves as the user's primary point of interaction. Holding this button for 3 to 4 seconds toggles the unit's power on and off. Tapping it answers and ends incoming calls, while a double tap redials the last called number on the paired phone. Pressing and holding the multifunction button for about 2 seconds (the instructions list the duration as "two beeps") initiates voice-activated dialing on the paired phone, if supported. You should definitely check to make sure that your phone supports voice dialing before choosing this speakerphone, because (aside from basic redial) the K100 doesn't feature any sort of dialing mechanism of its own.
The K100 pairs with compatible phones using the Bluetooth Hands-Free Profile (HFP) with a four-digit PIN for the handling of calls. The unit also supports A2DP audio streaming, but there is no support for AVRCP audio controls, so you won't get the normal play, pause, and skip commands that most streaming receivers offer. When connected to a handset that also supports A2DP, the K100 can serve as a loudspeaker for music, podcasts, and turn-by-turn directions from a navigation app. However, when combined with the K100's FM transmitter, the A2DP function becomes much more useful.
To the left of the volume knob is a button labeled FM. Tapping this button activates the K100's wireless FM transmitter, which deactivates the unit's loudspeaker and sends all outgoing audio over an FM radio frequency. Holding the FM button initiates a scan for the clearest channel, which, once found, is announced by the K100. You can then tune your car's radio to the same frequency and, ideally, hear the speakerphone's output through the stereo's speakers. Now the K100 can serve as a bridge between a Bluetooth music phone and a low-tech FM car stereo, which is quite useful when it works.