Not quite sure I understand what you want...
Each client, the devices you ssh from, has a key pair. You take the public key part of that pair and put it in the authorized_keys file on the server, the device you ssh to. The cannonical paths for your keys and authorized_keys is ~/.ssh/
You say you cannot reach your pi from your phone? Find the key file named something .pub on your phone, add it to the authorized_keys (as a single line in that file) on your pi. That is it.
If you also want to drop the username you need to add something like this to your .ssh/config on your phone:
Host alias_for_pi
Hostname pi_ipnumber
User pi_user
Does this help?