I bought a home with a Brinks system installed. How can I use the alarm without having it monitored by Brinks?

I believe the system is fully paid for so there is no issue there. I just want an alarm to sound if a door or window is opened without having to pay the monitoring fee. I have tried just plugging it in and setting it, which does work. But then after a while it starts beeping every few minutes. Can anyone help me?