Problems setting an Optima RF intruder alarm?

I have recently moved into a flat with an Optima RF alarm. Each time we try to set it we put the code in and it starts to beep (I presume to indicate the exit time) but within a few seconds the beeping changes to what I presume is the sound it makes when the alarm has been triggered. We have tried changing the exit time but it makes no difference. I was also advised when setting it to put the code in but not press set, however this just meant that the first beeping noise wouldn't stop. As far as I can see the lights flashing at various times are as they should be. Does anyone have any suggestions please?