I searched quite a bit and settled on Grasshopper.com. Main determining factors were price and web sip client support, i.e., make phone calls from your browser or mobile phone.<p>They also have an auto-attendant, multiple extension support, and voice mail to email capabilities.
I really like Grasshopper (<a href="http://www.grasshopper.com" rel="nofollow">http://www.grasshopper.com</a>), these days I use it for a second phone line for business-y stuff or things where I don't want to give out my cell.<p>Previously I had it at a consulting company, using the voice mail to email transcription and directory stuff (1 to John Smith, 2 to John Doe)
SendHub (a YC company, I think) seems like a good choice right now.<p>We left Phonebooth.com for SendHub and are fairly happy, though we're still waiting for a couple of features like which line was chosen from an initial menu.