GXW4104 and FreeBPX Fax Problem

Hello,

I have a Grandstream GXW4104 which handles 4 PSTN lines. It is connected to FreePPX, in which every channel is registered as a SIP Trunk.

Everything works fine according to phonecalls, both inbound and outbound.

My problem is that I cannot receive any fax at all.

I keep getting error

res_fax.c:1694 receivefax_t38_init: error on ‘SIP/GXW T4-00000608’ while waiting for T.38 negotiation.

res_fax.c:1946 receivefax_exec: error initializing chan nel ‘SIP/GXWT4-00000608’ in T.38 mode

GrandStream has T.38 FAX Settings in channels settings, where I have selected

mode = PassThrough
rate = 9600
ecm = Disabled

In FreePBX I have selected to work T.38 as Passthrough and have fax enabled to the specific extension,(enabled, fax email,attachment format).

Also, in my Inbound route for this line I have
Detect Faxes = Yes
Fax Detection type = SIP
and fax destination the specific extension.

Any help please?