If you use your cell phone as your primary business line, you really should know about Google Voice. You can use it as your primary phone service, as high-tech voicemail, as phone spam filter, and as advanced call routing, among other features. ZDNet has published a series of excellent articles breaking down the features of Google Voice and how to get the most of the service.
Start with this article: Google Voice: a cheapskate’s guide to cheap VOIP.
At this time, I use Google Voice for my personal voicemail, and I use Twilio for business calls so I can have more advanced routing and filtering.