tch mean holding time: average duration during which a TCH timeslot is allocated to a certain call.
for instance TCH MHT = 20s, it means that your subscribers get a TCH for 20 seconds. Then they probably handover to another cell, so the TCH in this cell is deallocated.
This is NOT the average call duration. You should look at the MHT along with the “HO per Call” indicator.
As far as i remember:
call duration = TCH MHT * (HOperCall + 1)