Why won't Chomp work on a prepaid phone?

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AC Question

chomp won't work on prepaid phone

Am using an LG Volt, was bought & used as a prepaid phone. To save money I have not bought a phone in a few months. Use wi-fi for everything & have been using a crappy free txt app. Any time I try to use a better app (chomp, handcent...) phone says I am blocked from using (even after set as default msging) ... Help?????