So your problem is odd. My first thought is that the cordless phone and the FXS port are not properly communicating or sending signaling information. But with that you would imagine that the voice port is off-hook and and when someone else called in they would indeed get a busy tone.
If this was me troubleshooting this I would start my eliminating a couple of things.
First is the problem the FXS port or the cordless phone?
Plug in a regular analog phone to the FXS port and see if you are still having the problem.
If problem is gone great! Then you know if is something specific to the one cordless phone.
Do you have another cordless phone to test? Maybe the base station is not properly talking to the wireless handset.
Can you provide us with the model of the cordless phone?
If you don't have another cordless phone to test or you already tested another one and same result move onto seeing if you can make any change on the port that may change the behavior
On the port itself you can try playing around with the signaling.
Is it loopstart or groundstart?
Don't know try both and see if you have any delta.
To do this
remenber when you make the change reactive the voice port
R1(conf-voiceport) no shut
Some other tools to help you out
show voice port
show voice port summary
show voice busyout
show voice dsp
Hope this helps out at all and let me know how it goes and what you discover along the way.
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