cannot recovery phone

[phil75]

Googmorning,
we have bought some month ago old tp28 phones.. my idea is to upgrading them to the latest firmware so I've download latest firmware and i've start an upgrade action... during this there was an elettrical issue....
now the phone is keep to reboting.. it seems to be hang...
so I did try to recover the firmware using tftp on boot...
So after modifying the default data i've renamed the firmware image to t28.rom and i've put it on my tftp root folder...
the strange thing is the follow:

on the tftp monitor i can see the request of file coming from the phone, but instead of getting the file called t28.rom it look for something called *****.bin, fs.img, yealink.img....
Why this?
How can i recover that?