I have been playing around with Asterisk and these are the settings that worked with both Slingshot ITalk and Snap Plus.
ITalk
Peer Details
type=friend
secret=XXxXXxX (Unique Password provided by Slingshot)
username=643XXxXXxX (Your Phone Number including 64 and areacode without 0)
fromuser=643XXxXXxX (Your Phone Number including 64 and areacode without 0)
host=chc.italk.co.nz
fromdomain=chc.italk.co.nz
dtmfmode=rfc2833
insecure=very
nat=yes
canreinvite=no
disallow=all
allow=ulaw&alaw
qualify=yes
User Details
type=friend
secret=XXxXXxX (Unique Password provided by Slingshot)
username=643XXxXXxX (Your Phone Number including 64 and areacode without 0)
fromuser=643XXxXXxX (Your Phone Number including 64 and areacode without 0)
host=chc.italk.co.nz
fromdomain=chc.italk.co.nz
dtmfmode=rfc2833
insecure=very
nat=yes
canreinvite=no
disallow=all
allow=ulaw&alaw
context=from-trunk
qualify=yes
Snap Plus
Peer Details
type=friend
secret=XXxXXxX (Your Account Password)
username=03XXxXXxX (Your Phone Number including areacode with the 0)
fromuser=03XXxXXxX (Your Phone Number including areacode with the 0)
host=connect1.plus.snap.net.nz
fromdomain=connect1.plus.snap.net.nz
dtmfmode=rfc2833
insecure=very
nat=yes
canreinvite=no
disallow=all
allow=ulaw&alaw
qualify=yes
User Details
type=friend
secret=XXxXXxX (Your Account Password)
username=03XXxXXxX (Your Phone Number including areacode with the 0)
fromuser=03XXxXXxX (Your Phone Number including areacode with the 0)
host=connect1.plus.snap.net.nz
fromdomain=connect1.plus.snap.net.nz
dtmfmode=rfc2833
insecure=very
nat=yes
canreinvite=no
disallow=all
allow=ulaw&alaw
context=from-trunk
qualify=yes
Some of these settings aren't needed for both, I just haven't had time so far to work that out
Asterisk needs to know the external IP by either DDNS or specifying if you have a static IP.
No Ports are needed to be forwarded for asterisk to connect to the ISP.