FXS interfaces are connected to voice-ports in CLI config.
voice-port 1/0 is the FXS 1/0
The led cards is on only during calls.
You can also use the command "sh voice call summary" to see the used port:
PORT CODEC VAD VTSP STATE VPM STATE
============== ======== === ==================== ======================
1/0 - - - FXSLS_ONHOOK
1/1 - - - FXSLS_ONHOOK
2/0 - - - FXSLS_ONHOOK
2/1 - - - FXSLS_ONHOOK
During calls the status will change to OFF HOOK.
Regards.