Can I use an AT&T iPhone 4S with no data plan? I have unlimited call and text?

I've had a really old phone since August 2010. This past summer, my family got an unlimited call and text plan. We don't need data, and I'm not that dependent on my phone. I just need to call, text, and use Wi-Fi whenever it's available.

Can I buy an iPhone off of eBay and pop in my SIM card? I've heard AT&T will detect that I'm using a Smartphone and charge me for it.
Please tell me if there's a way I can use an iPhone as if it's an iPod Touch that can text and call too. Thanks

Added (1). It makes sense because I wouldn't have to use a separate, old, and really hard to text with phone AND an iPod Touch, which is what I'm doing right now. I don't need data. But I need a phone that's easier to text with.

Likely not. The phone would be detected & the data plan applied. Buying an iPhone to be an iPod touch makes no sense.

Any mobile carrier will know you have a smartphone by the phones serial number (imei/esn) when you put you sim in the phone.

You can use a smartphone without a data plan with Net10 for $35 per 30 service days.

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