Apple Emergency SOS
iPhone users with iOS 11 or higher can use a feature called Emergency SOS. When you make a call with SOS your phone will automatically call the local emergency number. You can also set emergency contacts for your phone to automatically contact after you dial for help.
To use the feature, open your phone's settings then go to "Emergency SOS" to make sure it is activated. If the feature is turned on and you have an iPhone 8 or above you can press and hold the side button and one of the volume buttons until an Emergency SOS slider appears.
Slide the button to automatically dial 911.
If you are in a situation where you can't touch the screen, continue to hold both buttons then a countdown will appear and alert will sound. If you are still pressing both buttons at the end of the countdown, your phone will dial emergency services.
For iPhone 7 or earlier, quickly press the side or top button five times to get the Emergency SOS slider to appear. After the emergency call goes through, your iPhone can text people on your emergency contacts list to let them know you called for help.
Samsung SOS
Samsung Galaxy phones allow users to set up a feature that will text emergency contacts and share your location if you press the power button three times.
To turn it on go to Settings > Advanced Features > Send SOS messages.
In addition to designating emergency contacts you can also set the feature to attach pictures or audio that is recorded at the time of the activation. This feature sends messages and your current location to your contacts, but it does not call emergency services.
Other Apps
The app Kitestring does not require any particular phone, it uses SMS text messaging. Once you sign up tell the app when you'e out and it will send text messages to check in with you. If you don't respond the app will send an alert to your preset emergency contacts. The app is free for three trips per month and one contact. Unlimited trips and contacts are a $3 upgrade.
Circle of 6 is a similar app that was originally designed for college students to prevent sexual violence but is now used by teens, parents and friends to help keep one another safe. The app lets you pick six contacts who will get preset messages when you need help. It can be used to send GPS coordinates or request a call to get you out of a situation you no longer want to be in.
You can find more information about safety apps and devices at PCMag.