Google Voice is a free product/service that is perfect for business. It is a product that is revolutionizing telecommunications. PC World ranked Google Voice the #2 product of 2009.
The service provides a single phone number that rings all the lines you want. Free text messaging. It provides visual voice-mail. The service transcribes the message and displays it like an email. One can vary voicemail greetings by caller. These are several other great features.
If you do not already have it, go get it. It is free https://www.google.com/voice