There are many office phone solutions. One common need is phone extensions.
RingCentral Professional ($19.99 per month with 5 extensions and 1000 minutes) to grasshopper.com ($12 per month with unlimited extensions + 6 cents per minutes)
The other option for tech savvy is use netTalk and Obi110 and Asterik.
I want to call one important fact from the blog’s comments:
Why not just use the Nettalk SIP credentials directly on Asterisk without the dongles and the ATA (like many of us have done for years)?
[WM: We don’t recommend that users violate NetTalk’s express terms of service.]