Sam, sure – no problem, as it is in my expertize area, just ask!
Monem, the half-rate generaly is triggered by the BSC, depending on the features of the BSS and the according parameters. From my experience with couple of vendors, it depends of the TCH load of the cell (mean the percentage of idle TCHs) and if it is lower than some treshold – every next call setup is assigned a TCH/H.