Putting my SIM card in an old iphone?

My dad got a new iphone, so i currently use his old one (an iphone 3) as an ipod. However I was wondering if my SIM card worked in his, so I tried it out and it did (i got service right away). If I wanted to, could I use the iphone as a cellphone? Currently I do have a phone plan with AT&T, including unlimited data service for $8 a month (i got a cheap price because my current phone is not a smart phone). If I did use it, would AT&T charge me more money for using it? I just want to know what rate changes would apply.

No they won't really care as long as your using their plan and the phone is on a at&t service or is unlocked then there's no problem.
ne extra fees

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