ADB device list doesn't show phone

Finally got the device detected in adb under adb devices command by restarting adb server with root privilege as mentioned here in the Troubleshooting adb section.

sudo adb kill-server
sudo adb start-server

After this adb devices returned

7dd91912      unauthorized

with a prompt on device for RSA authorize. After authorizing

adb devices now returns

72291912      device

You installed the adb program correctly. The Android Debugging isn't activated by default. The activation option is hidden. You'll have to reveal it with these steps:

Goto Settings -> About phone -> (scroll to) Build number ->
 (Tap on it about 7 times) -> (You should get a notice
 You have enabled the development settings)

Now you will have a new menu item in settings called Developer options.

Go into this option and turn on the USB debugging option.

Now when you plug in the USB cable to the computer, you'll get the prompt to Authorize the connected computer.

After the debugging is turned on you may have to restart the server. You can do that with these commands:

$ adb kill-server
$ adb start-server

Make sure you have a good USB cable and are plugged into a good port. This was always the problem I experience when using ADB and fastboot. Also make sure you update and have the latest ADB drivers.

Gilberto Albino gave me a clue.
Nobody tells you that some usb cables are only good for charging devices, not for transferring data. You have to figure out which you have for yourself.
I think the charge-only cables are thinner.