Will my iphone work with just putting in a SIM card?

My old phone was turning off all the time and my mom had an old iphone 3 that she doesn't use anymore. I took her iphone and put my SIM card into it. I then plugged it into the computer and registered it with itunes. Will this charge my AT&T bill? I don't want to pay for data yet? I didn't activate it through AT&T, but now I'm kind of worried it will. Will the phone work okay without calling AT&T to activate it?

Yes it will just change the sim card and your line will be the same as before. Just a different phone

The phone is already activated and if you're getting signal on your phone, it's ready.AT&T will charge you for data as a data plan is mandatory for smartphone.

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