The Firebrick is a back to back user agent, and on OnSIP must be configured for OnSIP Trunking.
Before you configure the Firebrick, you must first create your account with OnSIP and make two minor modifications.
First, login to the OnSIP Admin portal and click on "Users" on the left side of the screen. With the user you create in OnSIP, make sure that under "Inbound Call Preferences" that "OnSIP Trunking" is checked.
Secondly, under "Phone Configuration" in the lower right, note your "Auth Username." Select your Auth Username and copy it to the clipboard or simply write it down. Next, choose "modify" under your "SIP Address" in the upper left corner of your user. Modify your username to be the same as your Auth Username (found under Phone Configuration).
For example, if your SIP address was email@example.com and your Auth Username was "yourdomain_user", you would change your SIP address to be firstname.lastname@example.org. As soon as you hit "save", you should see both the Auth Username and Username to be identical in the Phone Configuration section of the page. If they are not identical, modify the SIP address again to correct any issues.
In the Firebrick you would then use your Username (the same as your Auth Username now) as the From username in the Firebrick configuration screen.