You're not paranoid. This is perfectly normal. Builders will never make their contact details available easily to prevent people from contacting them asking when their house is going to be ready, or if they can get an extra few plugs installed, of if the place will be swept out before they get to see it, or if they can get to see the place before the snag, etc etc etc.
Best chance of making some sort of contact is to go to the site itself and ask to speak to either the Foreman, or the Snagging Foreman.