flexfax: random "MODEM WRITE ERROR: errno 5"

["Giulio" <giulioo@tin.it>]

Linux 2.0.33, Hylafax4.0pl1, USR sportster28800 external, send-only
environment.

Randomly Hylafax fails to send a fax because it cannot communicate well with
the modem, then (without modifying anything) I resend the same fax with the
same sendfax command and the fax is sent ok.
faxstat -dl reports "Unable to configure the modem".

Basically the modem seems not to respond to Hylafax commands:

This is a log of a failed fax:
Jul 15 16:24:15.58: [ 2859]: SESSION BEGIN 00000293 39627670
Jul 15 16:24:15.60: [ 2859]: SEND FAX: JOB 321 DEST 0,627670 COMMID 00000293

Jul 15 16:24:15.60: [ 2859]: DELAY 2600 ms
Jul 15 16:24:18.71: [ 2859]: DELAY 300 ms
Jul 15 16:24:19.02: [ 2859]: <-- [24:ATE0V1Q0S0=0H0\r]
Jul 15 16:24:19.02: [ 2859]: MODEM WRITE ERROR: errno 5
Jul 15 16:24:19.02: [ 2859]: DELAY 300 ms
Jul 15 16:24:19.33: [ 2859]: <-- [5:ATM0\r]
Jul 15 16:24:19.33: [ 2859]: MODEM WRITE ERROR: errno 5
Jul 15 16:24:19.33: [ 2859]: DELAY 300 ms
Jul 15 16:24:19.64: [ 2859]: <-- [12:AT+FCLASS=1\r]
Jul 15 16:24:19.64: [ 2859]: MODEM WRITE ERROR: errno 5
Jul 15 16:24:19.64: [ 2859]: SEND FAILED: Unable to configure modem for fax
use.....................and continues this way

What can I do?
The same modem work perfectly for outgoing data calls.

TIA.
Giulio.