try changing the codec on your trunk to g711 only

This changes de error to:

-- Channel 'SIP/xxxxxxxxxxxxxx' FAX session '26' is complete, result: 'FAILED' (FAX_FAILURE_PROTOCOL_ERROR), error: 'REMOTE_DISCONNECT', pages: 0, resolution: 'unknown', transfer rate: '2400', remoteSID: ''

That was without T.38.

With t.38:

-- Channel 'SIP/xxxxxxxxxxx' FAX session '7' is complete, result: 'FAILED' (FAX_NO_FAX), error: 'T1_TIMEOUT', pages: 0, resolution: 'unknown', transfer rate: '2400', remoteSID: ''

We had a similar issue only to find out our sip provider didn't support T.38.

But then I should be able to send faxes at least in G.711, don't I?

remote disconnect is normally what you see when your trying to do an in-band (T.30) fax over a codec like g729, you need to verify (via sip show channels) that your call is indeed g711.

Yes, it's g.711. I only allow this codec now. And the output of "sip show channels" confirms it.

what version of asterisk and what module version of res_fax and res_fax_digium ?

Asterisk 1.6.0.26
res_fax-1.4_1.2.0-x86_32
res_fax-1.6.0.14_1.2.0-x86_32

I have also tried with Asterisk-1.6.1.18

Sorry it was

res_fax-1.6.0.14_1.2.0-x86_32
res_fax_digium-1.6.0.14_1.2.0-barcelona_32

well at least you have the new version of the modules.. so when you open a case with digium they will be able to do a debug...

http://blogs.digium.com/2010/04/21/more-fax-more-features/

the only thing available for you to try on our end is in special_features.include

under [tl-faxreceive-faxopt]

comment out this line

exten => s,n,Set(FAXOPT(ecm)=yes)

to disable ecm which only happens on an in-band fax