One important piece of information regarding Cisco phone images relates to the image version and authenticity checking. Check your firmware version to see if this applies.
The information below relates specifically to V5 firmware however it applies to all subsequent releases as well.
Cisco IP Phone firmware images now contain a
Cisco digital signature to provide authentication for
improved security. Once firmware version 5.0 is
loaded onto a phone, all subsequent firmware
versions will be checked for authenticity.
Firmware images prior to release 5.0 do not contain
a Cisco digital signature and therefore once version
5.0 or greater is loaded onto a phone, all prior firmware
releases will be rejected.
! This means that you will NOT be able to downgrade the
! firmware to a release prior to 5.0 once 5.0 or greater
! is installed onto a phone.
! Cisco recommends the following procedure
! for upgrading to release 5.3
Upgrading to Release 5.3
When you upgrade to Release 5.3, you will download
a ZIP archive instead of a firmware image file as in earlier releases.
Contained in the archive are the unsigned (.bin) and signed (.sbn) binary
Release 5.3 is a maintenance release follow up to Release 5.2.
The following issues have been addressed in Release 5.3:
SIP phone doesnt use the media level contact field
Step 1 Unzip the ZIP archive to extract the binary images and any notes or
readme text files. Read these text files for any special directions
regarding the images.
Step 2 Copy the new images P0S3-xx-y-zz.sbn (signed binary) and P0S3-xx-y-zz.bin
(unsigned binary), where xx is the release major version, y is the release
minor version, and zz is the maintenance number, to the root directory
of the TFTP server.
Step 3 Using a text editor, open the configuration file and update the image version
specified in the image_version variable. The version name in the image_version
variable should match the version name (without the .sbn extension) of the
latest firmware that you downloaded (for example, P0S3-xx-y-zz).
Step 4 Reset each phone.
The phone contacts the TFTP server and requests its configuration files. The phone
compares the image defined in the file to the image that it has stored in Flash memory.
If the phone determines that the image defined in the file differs from the image in
Flash memory, it downloads the image defined in the configuration file (which is stored
in the root directory on the TFTP server). Once the new image has been downloaded, the
phone programs that image into Flash memory and then reboots.
For additional release 5.3 information
see the admin guides and release notes located: