One way pagers only work in a particular area of the world. In that area all the transmitters send all the pages. Such systems do not scale all that well and as a result getting pager service over a wide area can be fairly expensive.

There's a few nationwide pager networks out there; they're quite easy to build without the task of making communication two way. Just set up a hundred or so watt UHF transmitter at an antenna farm, feed it with PSK or FSK from a satellite receiver, and boom! You just covered most of a large city.