You handled that rude remark just right
Most of the time, if the speech is slowed down, it can be understood. Otherwise, you may need to pass this customer to another worker.
The other day I spent 2 hours on the phone with a person from India who answered questions about our FAX machine. Once in a while I had to ask him to repeat his sentence, or slow down. We got the problem solved. He was very patient with us!