I finally used the TB as a phone last yesterday for a good length of time. As I was very impressed with the sound quality. Last night I was where there are not alot of towers to hit and it was still really good.
I have had my TB for over a month and have a chance to use it in both very strong and very weak signal areas. Yes, I know, digital either works or it doesn't, but weak signals and super strong signals can sure seem to cause glitches. The TB has worked very well in both type of situations. If you need some extra volume, there is a hearing aid setting that increases the volume setting. The sound quality seems to be full and not tinny. Overall, I am happy with the TB call sound quality.
This probably doesn't apply to you, but I thought I'd include it anyway. Call quality is very subjective, but I'm and old guy and wear hearing aids and to me, the sound quality is very good, both with and without the hearing aids. As mentioned above, I do have the hearing aid setting on however.