Great tip, it works.
If you absolutely can't talk to the principal but can get his voicemail, don't leave some long "Hey Jim, it's Dan from XYZ Websites, and I think I can really help with your..."
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Instead, be vague and sound like you have something important for him. Busy bosses don't like to waste time, so if they can find a reason not to talk to you, they will.
Don't give them one.
Instead, say something like "Hey Jim, it's Dan, you can reach me at 555-4444. Thanks."
It's hard not to call that guy back. It could be anything- good or bad.
But you have to be ready when he calls...