It took him a long time to get to what the issue was, that he only starts to think about how to explain the error well-into the repeated calls. It's like he thought the issue was so self-evident that he doesn't start to refine and break down his explanations of why it's wrong until too late. Keeping things within one unit (rather than swapping between $/¢) as you suggest may have been better, but after two hours on the phone he sounds like he's already at his wit's end. It's really frustrating, so if you do have the time it's worth listening to