Why is my messages appearing on another iPhone?

My mum gave me her old iPhone 4 because mine broke and I've put my sim card in but for some reason when I get a text message it appears on my mum's iPhone 5 we're on the same provider which is 3 but we have different numbers

On your own iPhone, go to Settings > Messages > Receive At. Tap Caller ID and make sure it is set to your phone number.

Optionally, on the other iPhones, go to Settings > Messages > Receive At. Tap your Apple ID at the top of the screen and choose Sign Out.

You both receive same "text" messages? But different numbers? Seems impossible… But you can receive same iMessages on multiple iDevices… It's because one apple id is set for imessage on both devices… You can stop this by removing you mom's apple id on iMessage in settings…

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