GrandCentral - One Number fo Life
While it doesn't have all the functionality of a virtual PBX, the price is right: it is currently free. And it works.
I've been trying it out for a while and felt comfortable enough to make it the publicly listed phone number for my business. When I'm mobile, I forward the number to my cell phone. When I am at home, it is forwarded to my VoIP phone line. When at a client site, it is forwarded to the most convenient desk phone.
When I can't answer the phone, a voice mail is emailed to me.
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