The Vista 20P series of home security devices can be a bit complicated to understand as a product, since it was published under two different brands, the Ademco VISTA-20 and the Honeywell Vista 20P, both names represented the same device, with only one difference – the market where they were sold. So regardless which model you own, you can follow the same setup instructions and the same troubleshooting guide, both included in the Ademco VISTA-20 User Manual  which came with it. In case you lost it, or need a PDF version of it, you can download it from the monitoring center, which has it available for free: