Identifies the slot containing the line call was
received or sent over. It's value ranges between 1
and the value slotNumber in the 'slots' group. This
variable is equivalent to slotIndex in Ascend's
slot group. Applicable only if 'eventType' is
callOriginated(1), callAnswered(2), callCleared(3),
or baudRateChanged(6) otherwise 0 will be returned.