What I actually say:

• "he/she is not available at the moment."
• "they're away from the phone right now."
• "they had to step out for a moment."
• "he/she is on the other line right now."
• "they're in a meeting."
• "he/she is assisting another client at the moment."
• "they're out to lunch for the next hour or so."
• "all their lines are in use at the moment."

What I'd like to say:

• "he/she doesn't want to talk to you."
• "there's probably a reason he/she never returns your call."
• "they're too lazy to take their own calls."
• "no, it doesn't matter how many times you call. They never answer their own phones."
• "he/she does not know how to operate a telephone."
• "your reason for calling is absolutely ridiculous. Think of something legitimate and you might actually get a return call."
• "I have no idea where he/she is. I'm just making things up every time I answer the phone."