If someone is listening to your voicemail greeting it's only because they intend to leave you a message. Don't make them listen to 3 minutes of rambling in order to do so. Plus, you're more likely to deliver a clean, professional read if you stick to shorter greetings. If more specific information is needed, direct callers to where they can find it on your website.
While recording, it is a good idea to keep the iPhone voicemail message short. Ideally do not make callers listen to you for more than 25 seconds.

7. I can’t come to the phone right now because I’m down in the basement printing up a fresh new batch of twenty-dollar bills. If you need any money, or if you just want to check out my handiwork, please leave your name, number, and how much cash you need after the tone.
Our patented technology allows users to WRITE out their greeting script using keywords from their phone’s address book such as ‘FirstName’, ‘LastName’, ‘Number’ and ‘Company’. Once the user writes the greeting text, they choose a Voxist voice that will speak this greeting for them. The greeting they’ve written can apply to one or more callers that they designate.