"Hello, you've reached [name] at [company]. I'm unable to come to the phone right now. Leave your name and number, and I'll return your call as soon as I'm free. Thank you."
Think through what you are going to say before you place the call. When men hear the beep, they often freeze up and forget what they wanted to say. If you want to be the go-to guy at work, then you have to exhibit competence on all levels. Don’t spoil your confident image by hemming and hawing and filling your messages with “uhhhhs,” and “ummmms.” If it’s a really important call, consider writing out what you want to say beforehand. If the person picks up, great, you now have some notes to remember what you wanted to talk about. If they don’t answer, you can leave a clear and concise message.

Phrases like, "Please call me back when you get this," "I'm really looking forward to hearing from you," and "Call me at your earliest convenience," are pushy, aggressive, and borderline desperate.
In sales, immediate communication with your prospects and current customers is important. Being available to answer questions or further negotiate a deal allows your clients to feel secure and more apt to purchase from you. But what if you can’t answer the phone when they call? Make sure you have a voicemail system set up so your clients have the opportunity to leave you a message.