Trunk Settings

Add a SIP Trunk. Don't put anything in the Outgoing Dial Rules.

In the Outgoing Settings / Trunk Name put in FreeDigits. Enter the following into the PEER Details. Incoming Settings is not required.

PEER Details
allow=ulaw&alaw
context=from-pstn
disallow=all
dtmfmode=rfc2833
fromdomain=freedigits.net
fromuser=YOURNUMBER
host=freedigits.net
insecure=very
nat=no
secret=PASSWORD
type=friend
username=YOURNUMBER

The registration string that works for me is:

YOURNUMBER:PASSWORD@freedigits.net/YOURNUMBER

I tried it without the phone number at the end of the string but it didn't work. 

Now for the Inbound Route

On the left, click on the Inbound Routes. Enter your number in the DID Number box.

Down near the bottom of the page you set the destination where the calls are going.  Have it go to an extension, a ring group, a recording, or the conference.