iaxmode Adjusting skew

Hello,
I have installed hylafax and iaxmodem on trixbox Ce 2.8.03.. and I have used this guide to install these software:
http://www.trixbox.org/forums/trixbox-forums/share-your-trixbox-s...

I have configurated Eutelia SIP TRUNK to receive fax on IAX extension..

disallow=all
dtmfmode=info
allow=alaw
externip=X.X.X.X
context=from-trunk
fromuser=081XXXXXXX
host=voip.eutelia.it
insecure=very
nat=yes
realm=voip.eutelia.it
secret=XXXXXX
srvlookup=yes
type=user
username=081XXXXXXX

The incoming call arrive correctly:

-- Called hylafax-iaxmodem/123
-- Call accepted by 127.0.0.1 (format ulaw)
-- Format for call is ulaw
-- IAX2/hylafax-iaxmodem-16384 is ringing
-- IAX2/hylafax-iaxmodem-16384 answered Zap/1-1

iaxmodem initializes incoming fax correctly, but after few seconds i receive these ERROR logs:

[2006-04-04 13:42:22] Adjusting skew to -8150.
[2006-04-04 13:42:22] Adjusting skew to -8200.
[2006-04-04 13:42:22] Adjusting skew to -8250.
[2006-04-04 13:42:23] Adjusting skew to -8300.
[2006-04-04 13:42:23] Adjusting skew to -8350.
[2006-04-04 13:42:23] Adjusting skew to -8400.
[2006-04-04 13:42:23] Adjusting skew to -8450.
[2006-04-04 13:42:23] Adjusting skew to -8500.
[2006-04-04 13:42:23] Adjusting skew to -8550.
[2006-04-04 13:42:23] Adjusting skew to -8600.
[2006-04-04 13:42:23] Adjusting skew to -8650.
[2006-04-04 13:42:24] Adjusting skew to -8700.
[2006-04-04 13:42:24] Adjusting skew to -8750.
[2006-04-04 13:42:24] Adjusting skew to -8800.

and fax do not arrive..

Do you have any ideas which iaxmodem should be configurated to receive incoming fax on EUTELIA SIP trunk??
thank.