GXW4018 and HT818 inbound call from PTSN to specific fxs port


I managed to configure GXW4108 and HT818 to extend 6 analog trunks. I have configured port-to-port mapping for outbound calls using this guide http://www.grandstream.com/sites/default/files/Resources/gxw_wp_extending_analog_lines_0.pdf and everithing works as expected.
Now I need the same port-to-port mapping but with inbound calls (calls from PTSN->GXW4108->HT818). How can I achive this? I have tried different destination for each port using user id on the HT818. (ch1:101;ch2:102;ch3:103;ch4:104;ch5:105;ch6:106;ch7:107;ch8:108;) and associated each FXS port on HT818 with that specific user id. It does not work! It alwas uses FXS port 1
Any sugestion on how can I achive this??