Whilst BAU continues, I have more faith in PLBs, emergency radios, satellite telephones and the like than in homing pigeons.

Pigeons are perhaps worth considering in a post apocalyptic scenario. They are fairly easy to keep and can be bred for best homing instinct. Eat the failures !

O/T trivia, a major London hospital used homing pigeons until recently, to carry blood samples from the ward to a lab in another building.
The distance was modest, but pigeons were quicker and cheaper than a human messenger.