You do NOT need a data plan to use WiFi on the Epix or any other WiFi-enabled phone on AT&T or any other carrier for that matter

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However, most carriers, including AT&T, try hard to push a smartphone data plan with a smartphone, so you'll probably have to say "no, no, no" to the sales folks. Also, most smartphone rebates require a data plan, so you might not be eligible for any available rebates (read the fine print on any rebates to be sure).