Problems with t-38, fax and grandstream gxw4004

good morning

i have a question about the fax configuration (and the T.38 protocol) with the grandstream gxw-4004 gateway, with asterisk 1.4

At the moment our configuration is a double line :

isdn card -> asterisk server -> gateway gxw -> fax
fax -> gateway gxw -> asterisk server -> isdn card

Our criticity is the interface beetween the isdn card and the gateway :

fax receiving : ISDN (it works) --> ASTERISK --> GATEWAY (it doesn’t work) --> FAX

fax sending : ISDN (it doesn’t work) --> ASTERISK --> GATEWAY --> FAX

the gateway is ok because all the analogic phones connected works…
how we can configure it for fax?

thanks a lot