It is by design. It signifies an outbound signal connection was made successfully. My previous HTC Sense WinMo phone had a menu selection to turn it off/on. Can't find anything on the TBolt.
-Frank
Sent from my HTC ThunderBolt 4G/LTE using Tapatalk
I used to hate it, but now I got so used to it that when I'm using my wife's iPhone or my blackberry, I keep waiting for it to vibrate before I put the phone to my ear!