I'm still working on this issue, and decided to take a look at the openzap code.
First, I figured out that the parameter name for callerid is enable_callerid rather than enable-callerid.
I also figured out that this parameter defaults to TRUE (which is coherent with the observed behaviour on my FXO span)
By further checking the code, I figured out that presenting the callerid on an FXS port might not be implemented yet. I could see the code for retrieving the callerid from FXO but nothing to send it.
Is my asumption (feature not implemented) correct ?
François