I hope this helps anyone stuck with the same problem, the procedure I was following was correct. The bootloader linked in the IP 7000 support page is wrong.
The only way I managed to upgrade the phone was to download the software from this page:
Which link are you referring to so I can get this reported?
Please ensure to provide some feedback if this reply has helped you so other users can profit from your experience.
Polycom Global Services
I was having the same problem and was able to upgrade using the SoundPoint_IP_BootROM_4_4_0B_Upgrader_release_sig.zip file instead of the 4_4_0 version.