This is a long term known problem also known a muting issue. Restart your phone to clear, no need to pull the battery. I am hoping that this will be fixed with gingerbread. Many people have replaced their phones multiple time and still have the same issue. Costco replaced mine with a NEW phone and sim card about a month ago while under extended warrenty. I have not had the problem since. Don't know if it is a build vintage, firmware or network issues that cause this.
Wow I just noticed that you have a Bionic. My profile doesn't show it put I have had a Charge since March. I am normally on the Charge forum. If I was a betting person I would bet that Verizon has a serious network problem that is being blamed on the Samsung Charge phones