I was trying to configure SIP Trunk on my Elastix PBX I followed the various forums got configured today. I am sharing my experience .
1-On General Setting below are the configuration.
2- Dial Number Manipulation Rules leave it blank 
3-Under Outgoing setting
Trunk Name “nymgo”
—————————————
videosupport=no
type=friend
secret=your password
qualify=yes
insecure=port,invite
host=ata.nymgo.com
fromuser=your nymgo user name
fromdomain=ata.nymgo.com
disallowed_methods=UPDATE
disallow=all
directmedia=no
defaultuser=your nymgo user name
context=from-pstn
allow=g729
—————————————-
4- Incoming Setting Leave it blank
5-under registration
Register string
Nymgo username:- gukhan Nymgo Password :-12345678
Example :-gukhan:12345678@ata.nymgo.com/gkhan
For Trouble shooting SIP i got good article http://www.beardy.se/how-to-set-up-a-sip-trunk-in-the-asterisk-pbx