bell box failure?

I am an electrician and I gave installing my auntie an alarm a go anyhow a fault has occurred everything was working fine she went out the other day came back and on resetting the alarm she noticed the 24 hour light was lit so I went round to maintain the alarm and the bell box would not sound or strobe the two l.e.d's were flashing normally so I decided to power down the panel and start it back up the bell box rang for 5 seconds and then it stopped and continued not to work I tried this several times even looking the bell box tamper out didn't make it work