the panel could be programmed to delay the external bell on entry fail. unless you have the engineers code you could be snookered.
have you tried setting off a panic alarm of set the alarm off at the back of the house, ie. avoiding entry delay timers?
if you have a user menu, as with Texecom premier you should have the facility to test the bells by entering your code and pressing menu.
but again, unless you have access to the engineers menu your snookered.