A couple of years ago I got a few calls from people using their computers (in the days of dial-up, so it wasn’t the best quality).
Just wondering if any such services still exist. A google search showed a few pay sites and unrelated junk.
I remember doing it. 
WAG - It’s been surpassed by IP phones and corporate telephony moving into this in a big way. You might find some freeware/shareware out there, but I bet it’s no longer supported. Money talks. Big money screams.
At work our local office was the pilot for our federal agency (and the Department) using IP phones. We maintain a few POTS lines for emergencies, however, everything else is strictly IP phones. BTW, we use the same Cisco IP phones you see on the West Wing.
We’ve had our glitches, too. Cisco never designed their systems for faxes to use an IP network. They ended up visiting onsite and rewriting their code from scratch so fax machines would run across the IP network. I believe Cisco took that code back to their labs to incorporate it into their total IP system they now sell commercially.