In the cafe, you might tell your friend that you were going to 'pack your job in', but when you're writing to your boss, you're more likely to use 'resign'. I think it oversimplifies things, however, to say that phrasal verbs are informal. There is a tendency to use fewer phrasal verbs in formal language, but it is not wrong to use them, especially when they are part of professional jargon. If there's a one-word verb that is more formal , then use it, but don't think that a few phrasal verbs makes a text look colloquial.
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