This tutorial shows how to add your SDF VOIP extension as a trunk in FreePBX.
In the admin interface follow these steps, replacing
$PASSWORD with your extension number and password respectively:
host=sip.sdf.org username=$EXTENSION secret=$PASSWORD type=friend context=from-trunk insecure=port,invite trustrpid=yes sendrpid=yes directmedia=no qualify=yes keepalive=45 nat=yes dtmfmode=rfc2833 disallow=all allow=ulaw
You can now define Inbound and Outbound routes using this new trunk. To test inbound calls, you can queue a test call through