It's illegal to drive a car with earphones in both ears. This is why cellphone earsets are monaural (one ear). To make binaural (two ears) earphones for music that you can also use monaural on a cell phone, manufacturers have to do some tricks. Jabra's C220S is designed to have horrendous feedback if you try to use it binaurally with a microphone. Sennheiser's MM10 has an unique 4-conductor plug for use only with certain Motorola combo cellphones/MP3 players. The Plantronics MX100S has a simpler solution: two plugs. A standard 3.5mm stereo plug connects to any music player with a headphone jack; a standard 2.5mm plug connects to any standard cellphone. An in-line switch switches between the two. Simple, works perfectly. I use the MX100S with an anti-stuttering device that has both a 3.5mm headphones jack and a 2.5mm earset jack. I plug in both plugs and get binaural sound with the microphone. The microphone sound quality isn't as good as a boom microphone such as the MX150, but it's good enough. The earphones sound good. The rubber earclips hold the earphones in comfortably and securely, as good as the Jabra, and better than the Sennheiser. All in all, the MX100S works with no flaws, unlike the Jabra or the Sennheiser.