Keyholder monitoring is done through a company (the ‘receiving centre’), which will either respond itself when the alarm is triggered, or will contact your nominated keyholders.
Nominated keyholders must be within 20 minutes of the house, be able to drive and have access to the house. It’s your responsibility to keep the alarm company up to date with their details, and suggest alternatives if they are away. Alternatively you can ask a Keyholding company, the 24-hour service offered by monitoring, as well as knowing there is a dedicated company which is liable if action isn’t taken, may offer you piece of mind.