situation:
newer ANDROID devices doesn’t support USB mass-storage for file-transfer – you have to use the MTP (= Media Transfer Protocol)
solution:
1) install jmtpfs-package, e.g. apt-get install jmtpfs
2) connect your ANDROID device via USB cable and set the connection in ANDROID-popup as file-transfer mode
3) create your mount-folder, e.g. mkdir /mnt/phone
4) mount the phone to your mount-folder, e.g. jmtpfs /mnt/phone
Now you can browse the ANDROID filesystem under /mnt/phone