How to setup Fax machine + pots line on TDM400


I have a TDM400 with a single FXS and a single FXO (they are on ports 1,4 if it’s important)

I have outgoing calls working ok on the system using basically a few tweaks over the default zap.conf, but I have had no success whatsover making my fax machine talk down the pots line. Can someone please provide a working zap.conf for a setup which has a fax machine?

So to be clear the fax machine works down the pots line if I plug it in directly, and I just want it bridged when using asterisk. I disable all the echo cancelling but still outgoing fax calls have sometimes horrible feedback, or clear echo and I haven’t yet managed to get a successful sent

I guess something is wrong, but could someone please at least tell me the desired settings in zap.conf for use with a fax machine?

(This is in the UK if it makes any difference)