You need two phones, side by side to start. Open this page on both.
It plays and listens for tones, most too high for adults to hear (children and pets may hear them). Only numbers are sent: no recordings, no names, no location.
Pick one job on each phone.
1. Turn the volume to about half. On an iPhone, flip the side switch off silent (no orange showing).
2. Put this phone down and leave this screen open.
3. Go to the other phone.
You will hear a low note, then mostly silence: the high notes are there, too high to hear.
Hold this phone about one big step from the singing phone. You should hear a short low note from it every few seconds. If you hear nothing, the singing phone is muted: turn its volume up and its silent switch off.
Which phone is this? (optional)
On the singing phone, tap "sing one note only". Then walk away slowly holding this phone, and watch the number.
When it turns red, stop. How many metres (or big steps) away are you?