just to laydown some steps:

1) preliminar operations (to do only once)
mv /home/user/.local/share/sounds/Terrestial.aac.wav /home/user/.local/share/sounds/Terrestial.aac.wav.bak
mkfifo /home/user/.local/share/sounds/fiforingtone.wav

2) when someone is calling
mplayer -ao pcm:file=/home/user/.local/share/sounds/fiforingtone.wav ringtone.mp3 &
PIDMPLAYER=$!

3) after answering or closing the call
kill $PIDMPLAYER