Usually there are some timers and the drop is counted after they expire. The timers values has to be very carefully chosen, as there are a lot of rules concerning them. If you use siemens I can go further. Values are vendor specific.
in siemens statistic interface, is there a specific counter for the sdcch drop due to timer expire? in other words, does the siemens devices count how many a timer is expired? we dont have it in ericsson as i know