Assuming it's the same as here in the U.S.
field 503 should have the fax extension # in it.
You may need to increase the time in field 505,
this is the amount of time the disa card keeps detecting DTMF tones or CNG after the out going message has played.
Field 514 should be set to 1, this is the number of times the CNG has to be detected.
Field 510 tells the system what to do if there is no CNG or DTMF, it's probably set to intercept in which case you more then likely have the fax extension entered in intercept extension field.
Check all this out and we'll go from there.
MrG