Switching to iPhone.extra charge?

I currently have a cheap at&t phone and am on my mom's phone plan. My dad just upgraded to an iPhone 5 and doesn't need his iPhone 4 anymore. I was wondering if i switched to use my dads old iPhone would it raise my moms phone bill? My dad said as long as i don't use internet etc outside of Wi-Fi that it won't go up. Is this true or will just switching to the iPhone up the bill?

AT&T forces its users to a data plan when they start using their sim on a smartphone, They will automatically upgrade your basic phone line to a smartphone line with 300MB of data, which will cost you like $15.

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