I use pfsense running on an old PC running some kind of AMD CPU. 1GB memory and just a 2GB thumb drive. Traffic shaping for VoIP is the very reason I'm running it. My ADSL line has severe buffer bloat (
http://en.wikipedia.org/wiki/Bufferbloat) which basically kills all connections when someone is uploading anything.
I just ran through the traffic shaping wizard and gave it the IP address of PiaF server when the wizard asked about VoIP. You do have to make sure you set your outbound limit to just under what your uplink speed is.
I do have one weird very intermittent problem with the communications between PiaF and Vitelity. The setup will be working great for months, then out of the blue (no changes or updates on my end), I'll start getting failed call notifications from Vitelity (outbound calls work fine). I have to go into pfsense and clear all the states associated with the IP address of my PiaF box, then go into PiaF and do an "amportal restart". I'll have to do this every two or three days for maybe a week, then it goes back to rock solid. I assume it has something to do with tweaks Vitelity is making.