When dealing with difficult calls, put yourself in the caller's shoes, stay calm, and focus on the facts. Usually, angry customers are not objective, so before offering solutions, let them vent about what is upsetting them; this way, you defuse their frustration. Also, don't take their complaints personally; try to think of them as helpful feedback to improve your relationship with customers.
Take a look at this video below to know more about defusing frustration: https://www.youtube.com/watch?v=WphIXqTp_es
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