Just having it on.
How I noticed this was mine would change. THe first time was when I went to take off and could not get my engine speed up. Found out that the throttle was not set to open all the way. I now keep a back up program in my Compac.
What made me think of the phone is that when I am in my car, my radio gets static before the phone rings. It also gets static when it changes towers or power settings (bars change). I don't think it is the TX or RX signals but the intermediate signals that the phone uses that causes the problem. I now put my phone on the bench and away from my TX when it is on and I have not had a problem in over 18 months. Before this, I had 3 unexplained changes in 12 months. This covers the 2.5 year span I have owned my 9C.
ORIGINAL: sparkling_fist
by "using" your phone do you mean making a call or just having it on?