How can you be so sure it will be the ethnicity you call with? I'd bet that whoever was the last to cancel the order, regardless of the accent, would get the most hostility considering they would be the forth person to cancel an order. The first person on the otherhand, would probably get a more understanding tone.
It also depends on the person you're calling: One person may get mad regardless of who cancels due to their own anger control, while another may be very level headed and remain calm. And furthermore, you would only be able to gauge a reaction based on what they sound like - they could feel hostile but keep an even tone. Or they could have a tone you interpret as angry, but is really indifference. Or, just as you cancel, they may be angry about something else, and the pent up anger is taken out on you regardless of the accent.
And this is also on top of the fact you're calling a professional environment, they get paid to deal with these matters and certainly the cancellations may upset them, but it may not affect them whatsoever or they may be able to move past it. And those are just several, very simple factors that go into this "test". It is more than a matter of what you sound like.
And besides that, a decent amount of people could still tell by what you sound like even with an accent, so they would probably discover you were prank calling them.
And besides THAT, you are making them waste perfectly good food which could be going to somebody else that will actually pay them for the trouble that went into the meal and for their kind services. :c
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