I get that you have busy days and high call volumes. I get that you might not know if Bill is in the office or not. But please, would you actually LISTEN to what I say instead of presuming you know what I'm calling for?
But when I call and ask, "is Bill available," that does NOT mean "please transfer me to his extension." It is a question. You answer with "I don't know, but I can transfer you to his extension," or "He's on the other line, can I transfer...." if your phone system shows that. If I truly wanted to leave him a message I would ask for his voice mail or send a damned e-mail.
LISTEN to what the callers are saying, dammit!