I use Xubuntu 16.04 and when I connect my Samsung Galaxy S5 nothing happens, neither in the phone nor in the PC. The phone charges, yes, but nothing else. It does not appear any message or option about what to do with the USB connection (only charge, MTP, etc). I have installed gMTP and nothing.
Using the same phone, the same PC, the same cable, the same USB port but booting with Windows 8 everything works ok. The phone asks what to do (only charge, MTP, etc) and the PC can access normally to the content. This makes me believe that the problem is with Ubuntu.
I have noticed that when I plug the phone into the PC (Ubuntu) the "what do you want to do with the USB connection?" notification in Android appears and after about 2 seconds it automatically disappears. So it seems as is the phone does connect for a short time before it is unmounted automatically. I have tried "watch -n 0,1 lsblk" and also "watch -n 0,1 lsusb" to see if something appears during this short time period when it is plugged but nothing.