Here is my config:

Outgoing:

trunkname voipo.com

username=user
fromuser=user
fromdomain=east01.voipwelcome.com
type=peer
secret=secret
qualify=yes
host=east01.voipwelcome.com
disallow=all
allow=ulaw
insecure=invite

Incoming:

User context: VOIPO_in

type=peer
secret=secret
qualify=yes
insecure=very
host=east01.voipwelcome.com
disallow=all
allow=ulaw
context=custom-voipo

p.s. I may be off base here, but I seem to recall having problems if I didn't use 'peer' for inbound.