That is a feature called "post acknowledge", post acknowledge ensures user of the keypad that the signal has been received by the base station.
You can disable on individual keypad by pressing the clear (Clr) key and then press 920 on the keypad. Or set on/off for all keypads in selected ranges via PowerCom Keypad settings function. Please reference to PoweCom on line help topic"Reply World Wide Keypad Settings" for procedure.