The "virtual PBX" market is pretty crowded. Grasshopper is a good introductory service. There are some Skype/Gvoice-based options like Zaplee and PrettyMay. Moving up a bit you'd find RingCentral and Phonebooth which are a bit more capable.

Also, there are turnkey call centers, fully staffed. You get a number that gets routed to certain agents, and they answer as your org, have your flowchart/website/whatever. You're typically charged a setup fee, monthly fee, and per minute, but you don't have to go hire your own people.