In one sentence, describe what the following article is about:

Simply open a web browser on your computer and visit http://checkesnfree.com/. On the CheckESNFree home page, click on the “Select Carrier” at the center of the web page and choose the mobile network carrier the phone is on from the drop-down list. Next, type in your phone’s ESN on the text field provided below the “Select Carrier” option. Click on the “Check ESN” button to begin checking your ESN. Once the results have been displayed, it will tell you whether your phone’s ESN is clear or not.  If your ESN is clear, it means that you’ll be able to activate it on your current carrier. If the ESN has an issue, “Not Clear” will be displayed on the results in red letters. This means that you won’t be able to use the phone on its carrier. When the ESN returns as not clear, it means that the phone was reported to the carrier provider either as stolen or lost. In these cases, you need to contact your phone’s carrier’s customer service department to verify the current status of the ESN.
Head for the website. Select your mobile network carrier. Enter your phone’s ESN. Check your ESN.