Мессенджеры, которыми можно пользоваться без интернетаBridgefy - https://play.google.com/store/apps/details?id=me.bridgefy.main&hl=en
FireChat - https://firechat.en.uptodown.com/android
Briar - https://briarproject.org/
Two Way: Walkie Talkie - https://play.google.com/store/apps/details?id=com.selvaraj.twoway.android&hl=ru&gl=US
Hike - https://play.google.com/store/apps/details?id=com.bsb.hike&hl=en
Signal Offline Messenger - https://play.google.com/store/apps/details?id=com.raxis.signalapp&hl=en