For an Iphone:
Go to settings>general>about>scroll down to your model number. If you can not access your phone then look on the back cover and there will be some small writing on the bottom. Your Model number should start with an "A" and then have four numbers like "A1234"

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For an Android:
Go to settings>general>about>and scroll down to your Model Number. If you can not turn your phone on then take the back off and remove the battery and look for a white tab, the Model Number is usually located in the top left corner.