1. If you have a very small businesses, you could consider a solution such as gotvmail or virtualpbx. They supply you with one main number which can be programmed and extensions routed to cell phones or other numbers.
2. Another option is to outsource your phone system to a company such as M5 of Fonality which provides you with phone service over a high-speed Internet line. They provide the phones and everything you need and completely manage the system for you.
My .5? Well you could buy a PBX and manage it on your own - but not recommended. Also consider solutions from Talkswitch and Bizfon, which are mini PBX's and work with existing phone systems.
Editor and Technology Evangelist,