This is a great example of a traditional flash fiction structure where a bit of information revealed at the end adjusts the reader's interpretation of the rest of the story. (Dave Zeltserman's "Hostage Situation", which was the first in the Dissecting the Short Story series, has a similar structure, though it's about 2000 words in length and so is about twice the maximum typical length for a "flash fiction" story.)
Here, there aren't many clues along the way (at least, not that I sussed out) which will alert the reader to the possibility of the reveal at the end. Two which I noted after the fact were in Claire's reactions to things the customer said to her. When he tells her that he's not asking his unit to perform any "weird" tasks, she notes that "they" all claim that. While "they" could refer to customers, it can also be read to refer to "humans." A slightly stronger clue comes closer to the end. The customer asks why he should have to say please and notes "She's a damn robot!" Claire sighs "as quietly as possible" and notes to herself that the customer is "clearly prejudiced." Having the clues closer towards the end be stronger makes sense for this sort of structure.
As I discussed in my first point in "Five Tips for Writing Flash Fiction", it's important when writing at this length to stay focused. The entire story consists of one scene, spanning a handful of minutes, with only three speaking characters. The point of the story is to look at Claire's life in the call center and, in particular, how she has to deal with a customer who has an attitude about robots that Claire, as one herself, finds offensive. In under a thousand words, you have to stick to that point. A digression about the history of how personal robots came to be prevalent in society and the effect they have had on that culture won't fit. Every word has to be driving towards the main point.
Finally, the author notes which are presented with the story let us know that the author herself has worked in a tech support call center. I felt that the story had a good sense of realism, which her experiences undoubtedly helped with. My favorite little "nugget" along those lines was when the caller is yelling "But refusing to cook my bacon is not sweet!" and Claire notes that his head was turned away from the receiver and he was yelling at his robot, not at her. It's a little detail, but one I enjoyed.
Did The Story Work For Me?
Yes, I enjoyed the story. I didn't tip to the ending until it was right upon me, but when it arrived it was fully consistent with what had come before.