Can I use used iphone with my current sim card?

I got a iphone 3g from someone i know and I'm using att phone that is not a smartphone.
Can I use sim card from current phone I'm using and put it in the iphone I received and still use it with same monthly payment? Without paying anymore than I'm paying now?

Yes if the phone is The same carrier if not it will have to be unlocked.At&t and T-Mobile are gsm type phones and can be used by the other carrier after the phone is unlocked. If it's the same carrier then just put the sim card in and go.

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