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There’s an app called tasker that let’s you set up some pretty deep automation on your phone. I’ve used this a set up a grey list on my phone.
If a call comes in from a saved contact, all is normal. If it’s an unsaved number though, it doesn’t ring and goes to voicemail as if I just didn’t answer at all. If, however, they call back within two minutes, it will ring as per normal and I then know it’s, likely, someone who actually needs me.
I set this up because my phone number gets passed around at work quite a bit. If someone really needs me, I always figure they’ll call more than once or text after a missed call.
Likely durability and portability. Think of it as something they use month over month and just mark the day with something like a string band. Bone would be light enough to keep with you, strong enough to not break, and common enough to be available for household use.