So you have a tamper fault, simple. However finding the fault may not be that simple. Have you had any work done in the house recently that may have damaged the alarm. The next thing is how old is the outside bell box, could that be faulty or rusty inside.
If you are competent working inside your alarm panel then you need to check all the tamper pairs that come from all of your pirs, contacts etc. Be careful as there is 240v inside of the alarm panel. This may not be a DIY job if your not sure.
Hope this helps
Paul.