False "Busy signal detected" error

Conexant HCF PCI modem + Linuxant's commercial driver version 1.07
Hylafax 4.2.1
SuSE Linux 9.1

Receiving fax with Hylafax has been working flawlessly for a few months.
But a recurring problem happens when we try to send a fax: a "Busy
signal detected" is reported by the modem even when the remote line is
NOT busy.

Using minicom I can ATDT. But Hylafax can't. Do I need any special modem
initialization string?

Here goes a Hylafax session transcript:

[12216]: SESSION BEGIN 000000240 352226975647
[12216]: HylaFAX (tm) Version 4.2.1
[12216]: SEND FAX: JOB 21 DEST 226975647 COMMID 000000240 DEVICE
'/dev/modem'
[12216]: STATE CHANGE: RUNNING -> SENDING
[12216]: MODEM input buffering enabled
[12216]: Apply DialString rules to "226975647"
[12216]: --> return result "226975647"
[12216]: <-- [14:AT+FCLASS=1.0\r]
[12216]: --> [2:OK]
[12216]: MODEM set XON/XOFF/FLUSH: input ignored, output disabled
[12216]: DIAL 226975647
[12216]: <-- [14:ATDT226975647\r]
[12216]: --> [4:BUSY]
[12216]: SEND FAILED: JOB 21 DEST 226975647 ERR Busy signal detected
[12216]: <-- [5:ATH0\r]
[12216]: --> [2:OK]
[12216]: MODEM set DTR OFF
[12216]: STATE CHANGE: SENDING -> MODEMWAIT (timeout 5)
[12216]: SESSION END


Thanks,
Mark