If you're a do-it-yourself kind, you can set up an asterisk
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box that can handle just about anything you want. I use mine for my house with one incoming PSTN line (uses an FXO card) and multiple extensions (use FXS's).
You need a PC running linux (though I've heard someone has ported it to Windows) and as many cards/modules as you want extensions/incoming lines. If your business is big you can even go to PRI/T1 lines if you want.
For a 1 phone line, 3 extension system you'd need 1 PC running linux $500 or less 1 TDM400P with 1 FXO module and 3 FXS modules $313 1 Acquisition, installation, configuration of asterisk $0/your time
I use mine mostly to talk with relatives who use softphones on their PCs (which are free and free to use) and for those who don't I use a VOIP provider (currently I'm using nuphone.com and iconnecthere.com).
It takes a bit of work, but it's quite versatile and can handle a large number of calls. People use these boxes for calling card systems.
-- John