Buddhahint, I'm guessing that you racked up your overage before the $200 cap was instituted.
Tyler, the tethering package is a requirement, per terms of service, in order to use your phone as a mobile hotspot. Use outside of that package could get your service suspended or cancelled. For exact info, there is a "legal" link at the bottom of the U.S. Cellular web page.
Generally, security is set up when using the hotspot feature. When you turn it on in Settings > Wireless & Networks, you will see the SSID (access point name) and password or be allowed to create one, depending on the device. I highly recommend AGAINST setting your phone up as an "Open" hotspot, as then anyone within range could connect to it.
Samsung r880 Acclaim/i500 Mesmerize
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