Earlier quoted context omitted.
That's the reason to have your DNS at at least two different companies, working in tandem. In a case where one is down, your Unicorn Corp doesn't go down with it.
How does that work in practice? Even if I set NS records pointing to two different DNS providers, I don't think a DNS client would automatically switch and retry if one is too slow to respond/times out.
Most DNS resolvers will automatically try each NS record until they get a response. That might be your ISP rather than your iPhone but that's an old practice going back to when the internet was even less reliable because some random Sun box under someone's desk failed.
Modern web browsers will also do the same thing if a query returns multiple A records and they get a connection error.