Your phone cannot be used as a USB by itself. Both the phone and the computer must have an OS running on them already to utilize the MTP protocol, which allows you to transfer files between them.
Edit: A possible approach:
Warning, this is not secure and kind of defeats the point of Tails!
In this scenario, the only way to boot Tails from a phone would be to first boot a different OS on the computer, plug the phone in and mount it using the MTP protocol, then boot a virtual machine image stored on the phone with QEMU or similar.