Accenta 8 mini bell rang ?

I had an accenta 8 mini home alarm. Recently, the outside bell often rings when the alarm is not engaged. The keypad cannot stop it. I have to use the Tamper button (attack) to activated the all system, then I release the Tamper button. After this, the system allow me to use my code to stop the bell.
The fuses in the box seems ok, the battery is reccently replaced.
Would you have any suggestion please?

James
December 2015