PolyAI Ltd is an ambitious startup that creates artificial voices to replace call centre operators. Based in London, it has raised $28 million to bring AI-powered customer service to Metro Bank, BP and others. The idea is that instead of the nightmare of dialling random digits in a decision tree, you can instead ask to, say, book a table, and a voice—with just the slightest inflection of its machine-learning origins — responds with great civility. That’s nice. But there was a brief moment two years ago when it wasn’t polite at all.