Hi,,Whenever my alarm goes off, its silent and the siren/horn does not go off like it use to. I only know?

that the alarm has gone off when i get a call from the alarm monitoring company. The main panel is modelC1550 which i believe is Checkmate and and there r 3 control panels in 3 different rooms, the control paneIs r from a company called DSC. tried to troubleshoot the problem,but was unable,,can anyone help. [...]