The Volumio database is designed to search locally attached and network attached storage when assembling your library. This doesn’t include ephemeral devices like phones which move in and out of the network.
As an example, if you did have music indexed from your phone then those tracks would not be available to your family when you are out. Also playing the music through Volumio when you were in would drain your phone battery.
If you want to play music from your phone then you can play via UPNP, airplay (if you have an Apple device) or Bluetooth (if you have a Volumio license). Another option (assuming that you have the rights to do this) would be to put the music files from your phone onto the local storage of your volumio device.