What’s on the iPhone stays on the iPhone, say the big posters Apple has put up, not once, but twice apparently. However, the truth is a bit different and what’s on the iPhone may not stay on the iPhone.
For example, it turns out that what’s on the iPhone only stays on the iPhone providing there is no-one nearby with an Apple Watch and the Walkie Talkie app.
Apple slipped up with the security settings on the app and it enabled people nearby to listen in on your iPhone phone calls. Oops!
I am sure there will be an update to fix this, but for now, Apple has disabled the app so it cannot be used (source).