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I just brought my phone today and it automatically answers incoming calls

Hi, Check the call forwarding setting in your phone and ensure that it is set to the condition that you want. Normally you have Call Forwarding set to disabled, which allows you to select 3 more specific options. Here is a link that shows the settings. Scroll down to Call Forwarding https://support.t-mobile.com/docs/DOC-73

Why show call full screen…. My phone version is marshmallow.. But call option backdated Kit Kat