All of Apple's modern devices let you say "Hey Siri," to bring up Apple's smart voice assistant.
It's supported by the iPhone 6s or newer and the 6th generation iPad or newer. It even works on other products like the Apple Watch, and it's a really convenient way to access Siri if an iPhone or iPad isn't nearby.
I've found that sometimes Siri doesn't always listen to me as well as it once did, however.
Apple has a way that lets you retrain Siri to recognize your voice, which means your iPhone or iPad should start to recognize when you say "Hey Siri."
I'll show you how.