This used to happen to me when I was using 3g. Since I gave up on 3g and use edge with wifi in my office I am fine. Keep in mind that if you stray out of your wifi area and edge kicks in you will not get calls if you are on safari or sending email. They will go straight to voice mail. 3G was supposed to fix this issue but the constant switching between 3g and edge was giving me the same problem.