Why does my iPhone 5 automatically hang up at 4 hours?

Well me and my boyfriend talk endlessly on the phone for hours but right when we hit the 4 hour mark both our phones hang up, we have the same phone and it says the samething when it hangs up "call failure" how can I change this? Or can it be changed? Also, why does this happen?

The phone might be getting over heated make sure your phone is not in a case or in a hot area when you talk that long

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