I had this once when I first got my 7100T, I think that at that time it was the 3.8 firmware so I don't think it is related to the revision. The problem was that it was a year and a half ago and I really don't remember how I solved it other than I don't remember having to update the firmware. In fact I can almost guarantee this because I was at home and I only update in the office. I think the problem was that I started a call in regular phone mode then switched to speaker phone and ended a call in speaker phone. The setting seemed to stick through power down/up (don't know if pulling the battery would help). I think I fixed it by placing a call and switching in and out of speaker phone mode a couple of times and them ended the call in regular phone mode. Give it a go and see if it helps.