Hi Essam,
Several questions to come to mind based on what you described:
++ Did you upload the firmware on the Subscriber? The firmware files are not replicated via dbreplication and they need to be uploaded to each node which has the TFTP service running.
++ What firmware are you trying to install, what is defined in the device defaults page and what is the existing firmware on the phones?
++ What phone models are you doing this for?
++ Is there any firewall between the remote site and the Subscriber?
Regards,
Harmit.