Here are some links to walk you through the process of setting up with T38Fax, in approximately the order you'll want to read them.
Free Trial Signup - If you're planning on porting in, choose a temp number for your trunk. Implementation Guides - Some are also linked from the popular ATAs page.
Recommended Firewall Settings - Directly connected ATAs can usually skip this part.
Recommended Fax Machine Settings
Number Forwarding - Useful if you want to use your T38Fax trunk before your port is complete.
Number Porting - You must activate your trial account to port in a number.