In my experience with wireless hands-free devices, the best ones are wired. The problem with Bluetooth connectivity and any other type of wireless connection is the signal quality and voice quality that is affected by any efficient and under performing products.
At least with a wired connection you can ensure that the distortion of voice quality is not from your hands-free device that just may be the result of your cell phone or carriers network.
We all live in the day and age were cell phones can be very unreliable and have horrible call quality. Unfortunately this is just reality the type of technology we have available the market today. At some point hands-free wireless technology might become better in the voice quality that carries, until then I suggest using a wired device.