I've used IfByPhone and Twilio for a little more than a year for building telephony apps that integrate with software products to improve workflows. You are looking for feedback on their virtual phone system offering which I haven't used first hand.

If your team is tech savvy, OpenVBX from Twilio (http://www.twilio.com/api/openvbx) might be a good option.

You might want to check www.phonebooth.com as well.

Good luck!