I think you could find the message and record a replacement. The problem I see is that the all circuits are busy message is used under a variety of circumstances so it may be played even if the phone is plugged in.
The English sounds should be stored here
look for files with the word busy in the name.
You changes might not survive and upgrade to Asterisk.
Under Settings, Route Congestion messages, FreePBX gives you a little bit of flexibility over which messages can be changed. You can change the default message to any other system recording for certain congestion events.