Yep, that will boot to the bootloader. What we be useful is to get a screenshot of your bootloader screen, or boot your phone to the bootloader and post what's on the screen - specifically whether the bootloader is "Locked" or "Unlocked" and whether the phone is S-ON or S-OFF.
Get the mini-sdk tools from here:
ADB Guide (updated 2014/11/11)
along with instructions to install.
Edit: I can't remember off the top of my head if using vol down + power takes you straight to FASTBOOT mode or not. On the bootloader screen the mode is shown below the info text lines in green - this will either say FASTBOOT or HBOOT. If it says HBOOT, then to load FASTBOOT mode if you look further down the screen to the words in different colour text the top one should be Fastboot. Use the vol down button to highlight Fastboot and then press the power button to select, this will load FASTBOOT mode. If you then plug the phone in to your PC, on the phone screen you should then see FASTBOOT USB displayed if the phone is recognised ok.
Reboot phone to bootloader and plug in to PC (so you see FASTBOOT USB on bootloader screen). Open a command prompt window in the adb/fastboot folder on your PC - if in Explorer hold shift + right click, and pick "Open command window here" from context menu.
Then type:
fastboot devices (this is to check fastboot connects to phone ok, it should also return the phone serial number)
If it does, then type:
fastboot getvar all
and post the info the command returns but scrub out your IMEI number and serial number. Need to check what the CID number is plus a few other things.
Edit: I've assumed you have a Win PC or laptop available for this.