It works for me sometimes, but not for me other times. I have no idea why, but sometimes it refuses to send a message, and will usually not send it for the entire call. But other times I will be having large back and forth text message conversations while on the phone. I have no idea why. Even in the same location (my house). I have noticed also that sometimes (more rarely) it won't receive them either while I'm on the phone and as soon as I hang up a few will come through. You're probably experiencing the same thing I often do. It works (sending) more often than not for me, but it's still near half the time that it won't let me <shrug>...
I really doubt sprint's tech support will be of much assistance
