If you have this kind of problem - phone won't boot, not showing anything on screen upon powering etc. - and you tried softreset (123456789*0#) and hard one (3491672850*#), then you should give this option a try...for this to work you would need:
- tftpd32 software ( http://tftpd32.jounin.net/tftpd32_download.html )
- adequate firmware files for your 7900 phone ( www.cisco.com )
- local power adapter for IP phone - power cube 3,4 typically.
You are going to use tftpd32 software for locally provision of firmware to IP phone and as a DHCP server for same phone with required option 150 in it.
- configure your PC with static IP address - some value from your test DHCP pool. If you are going to use, let's say, 192.168.10.0/24 pool, then you can give your PC - for example - 192.168.10.100/24, with default gateway also 192.168.10.100;
- configure tftpd32 DHCP options as per picture:
IMAGE 1 - DHCP SETTINGS
- configure tftpd32 TFTP options per your environment and put your firmware files in same folder, like I did on mine:
IMAGE 2 - TFTP SETTINGS
- configure global options like this:
IMAGE 3 - GLOBAL OPTIONS SETTINGS
- on main window choose your LAN adapter as interface where you would listen for DHCP requests - IP shown in window should be your static assigned (ie 192.168.10.100)!:
IMAGE 4 - INTERFACE SETTINGS
- ALWAYS RUN THIS TFTPD32 SOFTWARE "AS ADMINISTRATOR..." ON YOUR WIN 7 OR WIN 8 MACHINE!!!;
- put your phone firmware files (extracted!) in corresponding folder - in this example it's on C:\TFTP;
- connect your PC with IP phone directly - put Ethernet cable in SW port on phone - not PC!; don't give power to phone yet;
- do standard hard reset procedure - holding the "#" key down on the keypad - apply power to the phone, phone is going throw some "light sequence", release "#" when typically handset blink non-stop red (or something similar), enter key sequence as noted above - 3491672850*#;
- observe throw tftpd32 log window how phone receives IP address from your pool and then downloads all required firmware files - let it finish that;
- phone should reset and be working now...
P.S. I found one interesting thing recently - tried to upgrade 7906 phone with newer firmware (9.2.. something) but it was looping nonstop (blank screen, not booting etc)...then tried some older firmware (8.2...) first and phone accepted that older firmware without problem...after that phone accepted 9.2 firmware also! Interesting...