Not as good as the CT-10, but a reasonable substitute
Customer Rating: 
I purchased this phone used because I managed to break my Plantronics CT-10 which I loved...okay maybe that's a strong word, but I liked it a lot.
At the time, I did conference calls at night about 1-2 hours in length about 2-3 times a week.
For the most part, sound quality, distance from the base unit reception, battery life were all comparable to the CT-10.
But there were 3 things this unit that made this unit not as good as the CT-10.
WEIRD EFFECT ON MY WIRELESS LAPTOP:
I know nothing about radio frequencies or such, but the minute I would turn on this head set to make a call near my wireless Dell laptop, my laptop would lose the wireless signal. Once the phone was off, it was fine. This wasn't a fluke but quite repeatable, as it fascinated me. My house cordless phone did not have this effect.
NO AUDIBLE MUTE INDICATOR AND A FUSSY MUTE BUTTON:
When you are on a conference call for a long time, you get up and wander, sometimes you cough or want to blow your nose and knowing you are on mute is essential. On this unit I had to look at it to tell and with it's black on gray display it was only visible in well lit areas. The CT-10 (since it had no display) had a high pitch beep you would hear in the background to tell if you were on mute. Mind you I'm sure some found it annoying, but I would have liked to have the option to have that on.
On top of that, to get mute, you had to hold the button to get it to go on. It was after another feature, that I did not use.
PASSIVE CLIP:
The clip on this unit was the type you hook over stuff. Cheap and somewhat effective for a belt, but sometimes when you have night calls you are not wearing a belt, heck you may not be wearing pants. Having a clip that can be opened and closed like the CT-10 makes it possible to clip to a bathrobe or other garb more easily. A passive clip needs a more rigid material to hook to.
MINOR ISSUES:
This unit has a red light on the end of the microphone. It only tells you if the unit is on a dial tone or in use (blinking). It would be nice if it indicated mute as well. Since it is barely in your line of sight, I did not notice it initially. It is more of an indicator to others that you are on a call.
On both the CT-10 and this unit the CT-12, the base unit could not be near any other electronics otherwise you would get a strong buzz. Easy enough to fix.
I was quite pleased how far I could wander around the house though. Often I was outside gardening while chatting on the phone. Unfortunately, this is how the CT-10 got broken...
WI-FI KILLER, SHORT BATTERY, CAN'T BE HEARD
Customer Rating: After spending $[...] bucks for this phone I expected a whole lot more. This unit sucks and I will never buy another plantronics product again.
First off I work from home and get sick and tired of knocking out every wireless device everytime a call comes in. I had inteded on using my laptop on some workdays but this "Wi-Fi Killer" has me stuck using on my desktop.
Secondly, why do I have this phone when after only 1 hour of talk time I have to switch back to my handheld phone.
Finally, why is it every other caller tells me to "Speak Up" because they can't hear me. If this phone would last a full 8 hours of talk time I would probably get laryngitis from yelling into the mouthpiece all day.
Plantronics has made a high priced piece of crap pawning it off like it is some great headset. I'd have been better off buying a $39 dollar cordless phone with a headset jack.
