I connected mine, as you have, to use the phone, then went to the section to add a portable player.
I suggest you delete the previous owner's phone first, then press the button to add a device.
The screen then changes and states something like, a device is already connected, do you want to disconnect?
Press yes, then follow the familiar pairing instructions as for using the phone, and your phone's description will appear in the info.
Switch off and now when you start everything up, there will be two notifications of a bluetooth connection, and both the phone will work as will the music.