MP3 files on my phone?

[ohrenkino]

See here for the different ways to connect a smartphone:

Sony Android Walkman

Not that I said On some devices you can set the USB mode to "USB" (not FTP, USB Tethering, MIDI, PTP). In newer OS versions that mode has been disabled for security reasons. So if you don't find it you have to transfer the files with the dedicated program first to a PC, modify them there and then transfer them back again.

AFAIK the usb mode is not supported any more in current Android versions.
So you would have to copy the files to a local drive first, edit them there, then transfer them back.
Unless you see the smartphone storage with a drive letter, MP3tag cannot edit the files directly.