I definitely ran into what @franckdanard mentioned when trying to set up a couple of our Alcatel 4018EE sets. I had finally got it to work - as in I could make calls from it and it could receive calls. However functionality was gone, the screen just showed the extension or caller ID. I had got the necessary files from our current OXE through FTP.
My opinion is to get new phones that are compatible with FreePBX, though that’s understandably a lot of money. We have a couple thousand phones in our district, all Alcatel. Many of them are older phones that are not SIP compatible.