Maybe an optional app? Send the keepalive email and alert the trusted family members when the app stops pinging back.

This would at least indicate that the phone is turned off or lost signal. All of this should be configurable by the user including thresholds before alert, reply timeout, etc.

I think expecting a ping from the app every 24h is a sensible default. Most people already "call to see if their phone is dead". This just automates it.