panic button 
noun
A button for summoning help in an emergency: personal attack circuits are operated by panic buttons...
- The panic button consists of two buttons, red and green.
- The terrified cashier couldn't use a panic button to alert emergency services because the phone lines to the garage have been out of order for three weeks.
- Her home will probably be fitted with a panic button to summon police if she is attacked.
Phrases
press (or push or hit) the panic button