My Alarm Bell Box does not work - Help?

I have a “Control 5” house alarm where the external siren does not work when the alarm is triggered.

The alarm has been reset to factory defaults, and all the wiring has been checked to ensure that there are no breaks etc there causing the fault.

When the control panel is set to do a self check, all zones work correctly and no faults are highlighted, and the internal sirens work as they should, but the again the external siren does not sound.

A voltage check shows power getting to the external siren, so I know that it is getting power from the control panel. If the power is removed from the control panel and the battery back-up, the outside siren then does sound, so I know the actual siren component of the bell box does work, as does its battery back-up for it.

I cannot find any information on the net about these alarms, so I don’t know if they go under a different name now, though I expect many alarms work on a similar principal, someone might have had a similar problem and be able to offer some help or diagnostic checks that might solve the problem.

Any help anyone can give me will be appreciated.

Thank-you in advance.