This is basically worked for me. I just synced two phones to my car (Galaxy S5 and a Nexus 5) on my S5 I just went into my Bluetooth settings, found Sync in my list clicked the settings button next to Sync and unchecked "Media audio", this allowed the other phone to do music while the S5 was connected for calls.Maybe go to your phone's bluetooth settings and only allow it to use the phone functions, not "music".