Stay calm and do not take it personally
The customer is frustrated with the situation, not with you. A steady voice and a short acknowledgement set the tone for the rest of the conversation.
Ask what they hope will change
A question such as what outcome would help most turns a demand into a problem you can solve. Often you can fix it on the spot.
Offer what you can decide
If a refund, an exception or a quick fix is within your authority, give it. Tell the customer plainly what you are doing and when.
Escalate when it is truly needed
If the request is beyond your authority or the customer still asks, pass it on without argument. Never make a customer prove they deserve a manager.
Brief the manager before the handoff
Write what happened, what you tried and what the customer wants, so they never have to repeat the story. A smooth handoff shows the team works together.
Learn from the pattern
Count how often it happens and why. A cluster around one policy or one product issue is something to fix upstream.