T38 Fax Over IP with Cisco SPA122 How To Guide
How to Use the Cisco SPA122 ATA to Send Faxes Over IP
The first thing to do is to set your Fax baud rate to 9600 or 14400 if you are daring. Turn off ECM on your fax machine. Recommended Fax settings.
Setup your Cisco ATA
Read this to get preferred settings. The preferred codec is G711u in Amerika and G711a in the rest of the world. Some jitter adjustments, and you are done.
Finally Take the T38 Challenge Test Your Settings
I have tested this with Net2Phone and it seems to work fine.