I’d also say that it might be helpful for this sales agent to be using a script to leave a message. Most of the time when I listen to voice mail, I don’t have a lot of time. Messages that sound like the sales person is fumbling for words make it very frustrating for the message recipient.
My 12 y/o niece put on her cell phone message, "Leave your name and number. If I love you, I'll call you back." Although not earth-shatteringly funny, it was cute the way she said it. In college my roomate was a cop, he told me that the worst things you can say on an answering machine were 1) telling them your name, 2) telling the caller your number, 3) and that you aren't at home. Well, here's what I did... Thats probably good advice for a home phone, but at the same time, I don't think it applies to a cell phone really. I used to have a longer message aimed at keeping people from leaving messages ... but I hate getting voicemails for the most part. One summer I had my little cousin start singing take me out to the ball game, I cut him off after a few lines, but it was just my version of satire against parents who have their kids sing, and it was pretty funny.

Don't create any unnecessary complications by asking someone to call you back at the office when you're calling from your cell phone.
A busy greeting should tell callers that your phone lines are currently in use. You can direct people to wait on hold or leave a message. For example, “Hello, you’ve reached [company name]. Our representatives are currently helping other customers. Please stay on the line to speak with the next available team member, or press one to leave a message and we’ll call you back.”