my dealer said you can use any 2.5 mm cordless or cellular headset with this system. But I have gone through 2 different kinds, Plantronics and AT&T. Plantronics one people said they couldn't hear me. The AT&T one I couldn't hear anyone else. Is this headset thing true or do I need one with an amplifier?

sorry this should say Sprint K3

I know on some systems that use the 2.5mm jacks you have to disable the speaker. Thats how the Vodavi is.

Make sure you have the headset ket on and by all means, I have seen this a thousand times, make sure the damn thing is plugged all the way in. Sometimes it's tight.