Palm treo 650
The 650 seems to have a delicate headphone/mic jack. Dropping the phone, even gently, seems mess up that jack. I returned my first treo 650 as defective, and got a new one. The same thing happened to my new 650 after I simply compressed the phone in my breast coat pocket by holding my mail against my chest.
I can't get a new one because the replacement treo comes with no warranty of any kind. :(
My advice would be to find a PDA/phone repair service to repair the jack or diagnose the problem if it turns out not to be the jack.