I've used thinkingphones.com at my last 2 startups. They seem to have a good mix of price/value and will work with you. VoIP has it's downfalls, the quality o

I've used thinkingphones.com at my last 2 startups. They seem to have a good mix of price/value and will work with you. VoIP has it's downfalls, the quality of the call is a factor of your bandwidth, and you can still get some echo or noise issues from time to time, but offers you the most flexibility and portability. My other recommendation would be to get a toll-free number of your own that you control, and set that up yourself for call forwarding to your main number. This way, if you have issues with whatever provider you choose, you have some ability to re-route your incoming calls through another carrier without being beholden to whichever company is providing your phone service.