Help with my fm transmitter

I need help with my fm transmitter. It is a sirius direct fm relay. It goes into the back of my car radio and uses a channel because my car doesn't have an aux option. When it is plugged into my sirius starmate8 it overrides the radio and works but wen I plug it into my phone and try to play music nothing happens. It just plays the radio station. If anyone knows how to make it work or if there is an app that will make it work or anyone who had the problem please help.

Thanks