Pagers

There used to be systems following different standards.

One of these standards is called POCSAG and there are some information about it at https://en.wikipedia.org/wiki/Radio-paging_code_No._1. Devices only need a receiver, no transmitter, so it is possible to make devices that will not disclose one's location.

The standard allows wide-area coverage, but it depends on every deployment.

As far as I am aware, these messages cannot be confidential and there is no protection against fake messages.