How to lock the phone app on an iPhone?

I don't want my daughter to accidentally call someone when she is playing with her apps.

Put the phone on airplane mode, or you can turn on parental controls in the settings app.

You can restrict her to only one app using Guided Access in accessibility, but you can't only lock one app.

On the iPhone you have the option to make it so that whoever is using your phone has to stay in an app and is unable to leave the application without your password. There's not really any other way to accomplish what you are looking for other than Guided Access. It limits your device to a single app and lets you control which app features are available.

It is located in:
Settings > General > Accessibility > Guided Access… Turn guided access on.

To activate Guided Access after its turned on in the settings:
1. Open the app you want to run.
2. Triple-click the Home button.
3. Adjust settings for the session, then click Start. (Circle any part of the screen you want to disable. Use the handles to adjust the area)

Put iphone on airplane mode is good idea cuz the app can't be locked.