The headphone jack on your EVO is junked... The way the phone senses headphones vs headset is impedance of the connection. In a PROPERLY working jack, the Microphone input (3rd band on headset plug) is shorted to the ground connection on a 2 band cable (headphones or male to male adapter) which tells the phone that it is headphones or audio only output. On a headset, the microphone input see's a higher impedance from the microphone connection which tells the phone that there is microphone attached. NOW on your phone & many other HTC phones with the slanted weird jack, the microphone connection in the jack is junked where it does not make good contact so the phone sees a higher impedance & thinks there is microphone attached... Thank HTC for poor manufacturing & engineering, I had the same problem on my Hero. I fixed it by taking it apart & bending the headphone jack contacts to make contact.