giving iPhone to mom - how to activate her number and DE-activate mine

[d0tMAC]

We have 3 lines on our AT&T plan( me, mom, and dad), 2 of which are iPhones. Me and my dad have iPhones. I want to switch with my mom and give her the iPhone and I want to go back to a regular phone. How would I do so? We each want to keep our own numbers obviously. I took my SIM out of the iPhone and put it in her regular phone, and it works. However, putting her SIM into the iPhone does not work. When plugging it into my mac, it says I need to activate it. The only thing is that I don't want AT&T to think I have 3 iPhones and start charging $20 extra a month for the data plan on each line. Is there a way to DE-activate my number and the activate her number? Because I feel that if i just go and activate her number, we will be charged as though we have 3 iPhones when we only have 2. I hope you guys understand what I am saying. Is there a way for me to let AT&T know that the iPhone is no longer under my number, but under my mom's number? Please let me know. Thank you.

 

[Hooka]

Call AT&T and explain what you are doing. Unless the contract is tied with that same number.