My Treo 680 is GSM unlocked, running the AT&T firmware(custom rom 1) published here.

1. Reboot your 680
2. Connect an earphone
3. Press any key(not in the dialing program, just go to any program else, e.g., the laucher). You will hear a buzz in the earphone upon each key strike.

4. Lauch the dialing program, go to the on-screen dialing pad, press a number key. The first DTMF tone has lower volume than it should have. Go to other program and press some keys again. After playing the first DTMF tone, there won't be buzz any more in the earphone wherever you press the keys.

It seems that playing a DTMF tone will eliminate the buzz.

Would you please have a try and confirm this? Thank you.