In Case of emergencies while in Silent Ringer mode

Put them among Favorite Contacts. See This guide: About the buttons and switches on your iPhone, iPad, or iPod touch - Apple Support

Section:" What the buttons and switches do", point #3

Regards

Giulio

Anyone can call you while you have the device on Mute. The only difference is that you will not get a ring, just a vibration if you have that activated.