No CNG tone with asterisk fax server using t38modem and hylafax

Hi asterisk users.

I have set up a fax server (asterisk + hylafax + t38modem) connected to my ipbx via a sip trunk and T.38 protocol. As this server is a mail to fax or fax to mail server, it handles only fax calls.

It works OK and sends or receive about 200 faxes per day.

But… there is one fax number (maybe more ?) that this server is unable to connect to when sending. I can send a fax to this machine from an external fax machine, but not from the fax server. If I call this number from a regular
phone, it answers but I get nothing but silence. This leads me to believe that this particular fax number is using a line sharing device of some kind, and it is looking for a CNG tone before sending the call to the real fax machine.

If i dial my phone number from my fax server, my phone rings but if I answer I can’t hear any tone from the fax server. Silence again.

So I think that most of the time, real fax machines send CED without waiting for a CNG tone from the sender (my server) and it works, but since this particular fax number remains silent, and so does my fax server, nothing happens and fax sending fails with “no carrier detected”.

Since I know that every call going out of asterisk is a fax, is there a way I could force asterisk to generate a CNG tone when dialing ?

Any help will be greatly appreciated.