It is difficult to say for sure without seeing the config, however here is my two cents worth. You did not say whether or not you are using SCCP with your FXS ports. I am going to assume that you are not.
When the FXS port makes the call it may or may not find a dial peer to go out. If it does not find a dial peer to route the call it will ring busy. To resolve this you will need to create a dial peer that will route the call to the FXO port. This is done through translation rules. The below linked document details how to add the translation rule to force the analog port out of the correct FXO port:
http://www.cisco.com/c/en/us/support/docs/voice/call-routing-dial-plans/41887-specific-fxs-call.html
The only configuration needed on the FXS port is a station-id number (extension) and caller-id if you wish to use this feature. Below are samples from a config where I have this configured:
voice-port 0/2/1 <------- FXS port with analog device
station-id number 1952
caller-id enable
dial-peer voice 21 pots <---------Inbound dial peer for FXS
translation-profile incoming anl1952
answer-address 1952
forward-digits all
voice translation-profile anl1952 <--------Translation profile specified in dial peer 21
translate called 52
voice translation-rule 52 <--------Translation rule referenced in profile
rule 1 // /32/
dial-peer voice 32 pots <-------Dial peer redirecting FXS port to FXO port
destination-pattern 32T
port 0/0/1
The above configuration will send all digits from the FXS port out the FXO port. You can further control dialed numbers by creating multiple dial peers with the 32 predix.
Hope this helps.
Regards,
Sam
