Packet8's virtual office system does this for $50 a month, but you need 3 extensions. It comes with a lot of benefits though -- free conference calling, an IVR, etc. The biggest drawback is that you have to buy their VOIP phone.

I know there's other companies out there that probably do the same, but I've been using Packet8 for about three months now without any problems and an associate of mine has for about a year without issue.

You can also contract out to a virtual office building which can handle your phone calls and such for you and then route as agreed.