[Allezxandre]

On iOS, just hold the button you would use to turn the phone off, but hold longer, it will loudly count down to 0 and call emergency services. You can also press the side button 5 times and it will call without you having to hold the button pressed. Works on iPhone and Apple Watch.

More info here: https://support.apple.com/en-us/HT208076

[dankwizard]

And as well, if the FaceID doesn't work & the PIN is prompted for, "Emergency" is available in the bottom left.

[jdsully]

Press and hold doesn’t work if you have Siri enabled.

[Allezxandre]

It does, just hold the buttons you would hold to turn the phone off, not the button you would use to trigger Siri.

On FaceID phones that means holding both the Lock and Volume Up buttons.