1. Keep it calm, civilized, and matter-of-fact.
2. Tell the agent that you want a refund. If he or she tries to push a voucher instead, calmly repeat no, I understand that DOT regulations require you to give me a refund if I want one, which I do.
3. If the agent still says no, ask to speak to a supervisor, and repeat the statement above. If they still will not agree to a refund, tell them (still keeping it civilized and courteous) that you will be filing a DOT complaint and disputing the charge on your credit card.
4. if they still have not agreed to a refund, don’t immediately file a credit card dispute or DOT complaint. Just call back and hopefully you will reach a more sensible agent. But if not, make the complaint, file the credit card dispute, making sure to keep copies of screenshots of your reservation and the fact that the airline cancelled the flight. You will win the dispute.
i have successfully obtained refunds from multiple airlines, and only needed to go through all the steps above with United, when the first agent I reached flat-out refused a refund, but when I called back reached a much more reasonable agent, and got my refund.