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You've got a couple of different things to consider: first, states regulate contests individually, so you would need to check the state laws of every state that you would be running the contest in. Some states may allow some kinds of contests; others may not allow the same contests.
To find information about state laws, in Google, type in the name of the state and then "contest laws". For example, California's Department of Consumer Affairs regulates contests in California. Here are the laws on point: http://www.leginfo.ca.gov/cgi-bin/displaycode?section=bpc&group=17001-18000&file=17530-17539.6.
You would also need to check with the company that is providing your telephone service to make sure that their rules allow you to run this kind of contest; and that they would be technically able to remit the payments that you are asking for to you.
There are companies that specialize in running online contests; to find those, type in "online contests" in Google. Those companies may be able to provide you with some guidance on how to do this.