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.
The telephone system manufacturer assumes that everybody knows how to connect a phone jack so they show only how to connect a 3.5mm stereo plug and jack.