I just ran the following test: I called the EXP number, but when prompted, entered my girlfriend's PLT number. Here are the results <drum roll please>:

The automated system answered the phone and said "Welcome to the AAdvantage Executive Platinum Desk." At the prompt I said "agent" and got the standard message, "To get you to the right agent, please say your AAdvantage number." I said my girlfriend's number. Instead of being directed to an EXP agent, the system then asked if I wanted to speak with an agent about travel with the United States, Puerto Rico or Canada (at which point I knew I was not getting an EXP agent, as EXP agents handle both domestic and international itineraries). I responded "yes" and then was directed to an agent, who simply answered "American Airlines" without saying "Executive Platinum Desk." (I did this twice to check my results!)

So, again, there's no way one is reaching an EXP agent without an active EXP number. I suspect njvandy's dad may be confused because the automated system answers "Welcome to the AAdvantage Executive Platinum Desk," but it figures you out before letting you speak with a live agent. BTW, I suspect one could call the main reservation number or the GLD or PLT phone number, enter a valid EXP number, and reach an EXP agent, but I haven't tested that yet!