sip.conf: 

[mediant]
type=peer
secret=1000
qualify=yes
language=en
insecure=very
host=10.1.16.13
host=dynamic
dtmfmode=rfc2833
disallow=all
context=from-trunk
canreinvite=no
allow=ulaw
nat=no

[8001]
type=friend
secret=8001
qualify=yes
host=dynamic
;dtmfmode=rfc2833
context=sstest
canreinvite=no

extensions.conf

[sstest]
exten => _X.,Dial(SIP/mediant/${EXTEN})
exten => 8000,1,Dial(SIP/8000)