Try this; Connect the left channel from the MP3 plug to pin 4 of the RJ45 connector that goes to the phone system and connect the shield from the MP3 plug to pin 5 of the RJ45 connector that goes to the phone system and see if that works.I have just tried an old headphone cable, 3.5mm plug - shield to pin 1, left channel to 2.
This didn't work !!