How it works: The Cryonics Institute's monitoring app (the first official on the scene) is a basic Android (only) application that sets a timer for your selected length and then asks you if you're ok. If you do not indicate you're ok after some time, the system escalates to your contacts. You can also select to pause monitoring for certain times of the day.
You also have an SOS button that says, "Send a message for help!" You should be thinking about your contacts in terms of what they can do for you that doesn't involve text messages and phone calls. The system already presumably texted and called you (you should put yourself as the first contact), and you didn't answer. So your contacts should include people who have some other way of checking on you, such as by coming to your house.
High-level overview of the issues: The issue with check-in apps that base escalation purely on whether you respond when prompted or answer a call is that it's easy to incur "false positives" by missing the event and having the system escalate to call your friends and family. This price to pay may discourage the user from continuing to use the application. Having the ability to pause the system for some time of the day helps mitigate these issues, but I imagine users still encounter false alarms to their friends and family routinely.
How does the system stack up against other current options?
Summary: The CI Check-In app reliably provides a basic check and is better than nothing. It can also be used as a backup for some faster but less reliable systems.