Asterisk 1.6.2.5 t38 Audiocodes MP-202 B no incoming fax

hi all,
i’m having troubles setting up asterisk with Audiocodes MP-202 B
asterisk version is 1.6.2.5 (from ubuntu repos)

outgoing faxes and calls are ok
fax (brother 2820) —> ATA (Audiocodes MP-202 B) —> asterisk —> provider …

nothing to do for incoming faxes:
asterisk console doesn’t complain about nothing, call remains active (connected) for 30 seconds (test fax is 1 page doc) and then normally hangup.

suggestions? comments? hints?
thank you in advice.

here some details:

extensions.conf —
[receive_fax]
exten => s,1,dial(SIP/20,60)

sip.conf —
t38pt_udptl=yes,fec,maxdatagram=320
t38pt_rtp=no
t38pt_tcp=no

;this is the section for the ATA
[20]
type=friend
username=20
secret=20
dtmfmode=rfc2833
host=dynamic
insecure=invite
nat=no
canreinvite=yes
qualify=yes
context=from_internal
disallow=all
t38pt_udptl=yes
allow=alaw
callgroup=1
pickupgroup=1

; this is the section for my trunk (incoming/outgoing)
[trunk]
defaultuser=trunk
disallow=all
allow=alaw
allow=g729
context=from_external
insecure=invite
secret=secret
type=friend
qualify=yes
nat=yes
localnet=192.168.0.0/255.255.0.0
canreinvite=no
t38_udptl=yes