If you go to line number 106, you will see:
exten => dial-BUSY,1,Goto(busy,1)
Then in line number 115, you will see:
exten => busy,1,PlayTones(congestion)
That means that if a destination number is BUSY, it will Play a congestion tone instead a busy tone.
To fix that, you will need to replace the line number 115 to:
exten => busy,1,PlayTones(busy)