If you're looking for a fully hosted solution, here are some I'd recommend checking out:

http://bandwidth.com/hostedvoip/

http://business.packet8.net/virtual_office/

If you want to do something internally, http://switchvox.com is a good software option as well. It's built on Asterisk (so I guess it could be duplicated for free), but is point and click and very easy to use/stable. No Asterisk skills needed.

Those are the ones I have experience with. HostGator still uses Switchvox (running on VOIPo service) for almost 300 employees now with virtually no issues.