Why does my phone say iPhone unavailable

Somebody entered the wrong passcode too many times

iPhone Disabled,  Unavailable, Security Lockout,  

You can sometimes remedy this without a computer if on iOS 15.2 or later by tapping on "erase iPhone" , but you will still need the Apple ID password. It will WIPE the phone, no choice!

How to remove your iPhone, iPad, or iPod touch passcode without a computer - Apple Support

video

https://youtu.be/7s8-zLG9ySQ

 ios 15.1 and earlier:

If you've forgotten your iPhone passcode – Apple Support (UK)

To prevent it opening in your pocket or purse and disabling the phone you can turn off Raise to Wake in Settings, Display & Brightness and turn off Tap to Wake in Settings, Accessibility, Touch