I believe that what your talking about is the network signal..
Above is an image of the notification bar with 2 icons: the signal bar and the mobile network icon which is the icon on the left. That mobile network icon should now not be there any more because you disabled it. Now the icon on the right, the signal bar, shows you the signal strength and whether the signal is 2G or 3G. The difference between 2G and 3G is, 2G is the standard network connection, whereas 3G is the newer connection to the network, allowing you to have higher quality calls and fast speeds when using the internet, when not using Wi-Fi. As you have disabled the mobile network, your credit should not be getting used up any more