A web search for the company or the president’s name will often enough turn up obscure news items or papers with the president’s extension number.
Asking for “extension 5301 please” is more likely to get you through than “Could I speak to Mister Greaves, please?”.
In the first case you come across as knowing who you want and that you deserve direct access.
In the second case you are asking the receptionist if they would deign to do you a favor, and why should they? They don’t know you!