T38 passthrough issue when sending

Hi everyone.
Asterisk 1.4.1 rpm with NAT
ATA (Wellgate 3701B) with T38 connects to Asterisk (ext 401)
t38pt_udptl= yes both in sip.conf and extension

Receiving faxes: OK
Sending faxes: first page gets sent, then line goes down
In Asterisk full log a "“RTP re-invite after T38 session not handled yet” gets logged

If I set canreinvite=yes to canreinvite=no, nothing works (not even receiving).

The firewall statically nats ports 5060/5060 and 4000-4999 and 8000 to 20000 to the asterisk box.

Any idea where to look ?