How to tweak AC impedance on analog FXO ports

could anyone give the details how to set the AC impedance of analog trunk ports in the zaptel drivers e.g. wctdm.c?

i know it’s in the fxo_modes[].acim array. setting opermode=COUNTRY in the modules config file is not enough because our PSTN operators uses different impedance parameters. i want to know what acim values to set the impedance to 600, 900, etc.