If you want to go the other way, and have the phones ping the network instead of the other way around, there are applications that regularly publish to message

If you want to go the other way, and have the phones ping the network instead of the other way around, there are applications that regularly publish to message queues. The Android app Presence Publisher [1] for example publishes to MQTT. [1]: https://f-droid.org/en/packages/org.ostrya.presencepublisher/ https://f-droid.org/en/packages/org.ostrya.presencepublisher...