As tails is loaded via USB drive and your phone can act as a USB drive,.is it possible for your phone to have tails saved and show it when connected as a USB? It would be handy if you had it and other OS available wherever you went by just plugging in your phone.
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.
Basically any Linux distro within reason and with a few exceptions. I think some beginner / gaming distros add dumb crap. I like OpenSUSE but use whatever floats your boat.
Tails is a specific use distro, the idea is that with it you can access Tor and other services while leaving minimal traces behind.
Their homepage highlights activists, journalists and even domestic abuse victims using it to avoid censorship/surveillance and access the internet safely in unsafe places.