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. The alarm company want to charge me my both arms and my both legs, they want $75/ for first 1/2hr and then its $18.50 for every 15 mins,what a ripoff. Please help anyone.