While searching for ways to transmit Pager messages I found this nice post:
https://debugger.medium.com/howto-using-a-pager-in-the-21st-century-6a57454ecde8
There are many ways to transmit to pager using the POCSAG protocol, but I decided to buy a MMDVM Raspberry Pi Hat board: https://aliexpress.com/item/32915442246.html
I bought the model “MMDVM OLED CASE” without the “Raspi 0 W” because I already have one! Then I spent around U$ 30,00 with free shipping, very good!
When it arrived I mounted it over my Raspi board running Raspbian 11.
Basically I cloned and compiled the MMDVMHost and its dependencies like WiringPi and “ArduiPi OLED”.
After compiling and installing I moved the file MMDVM.ini to /etc/ and edited it to enable POCSAG protocol and “Remote Control”.
Unfortunately while running the “RemoteCommand 7642 page 1000001 Ola Mundo” it was returning this error message:
“Received a NAK to the SET_FREQ command from the modem”
It was fixed changing RXFrequency, TXFrequency and POCSAG Frequency to 433450000.
Unfortunately many forums has this issue but nobody explain the root cause, some report the issue was caused by incompatible MMDVM_HS firmware version, duplex config enable for this board that doesn’t support duplex, etc. The only place where I found the right root cause was here:
https://github.com/juribeparada/MMDVM_HS/issues/74
Because of it I spent time testing many configurations, updating firmware, etc. At least I’m now using a more updated firmware, from MMDVM_HS_Hat-v1.4.17 to MMDVM_HS_Hat-v1.5.2
Alan C. Assis 
In the pre-cellular days I wore a pager for work. My first ones simply beeped and you were to call the office to see what they wanted. Later on I dad a new alpha-numeric model which could display a brief ~20 character message. Other than for doctors the pager days seem pretty dead. Is this a “just for fun” project or do have some real use for a pager?
Very nice, I never used a Pager before. That is true, only some hospitals still using it. I want to use it to receive lat/long from a balloon and find it easily. Pager technology is easy and reliable
I never thought about using POCSAG for a location beacon for balloons, drones, etc. Good project idea. BTW, the company that I worked for hired a university research lab to perform a study of RF penetration of buildings based on pager frequencies in use withing the USA. The winner for best penetration was 406-422 MHz.
Interesting! Did they tested lower frequencies? Below 400MHz? Normally lower frequencies can reach deeper inside buildings
The company owned cellular and pager services and they tested all pager frequencies as well as cellular frequencies that they had licenses for. They tested from 157MHz to 1.8GHz bands. Those in the 400MHz region penetrated buildings the best. Penetration drops rather rapidly as you go above 900MHz. The company also used frequencies in the 7GHz range for microwave and even rain or fog will stop those signals. the higher that you go the more RF behaves like visible light.
Hmm, I used to think as lower is the frequency better it will pass through the walls. Maybe it explains why 433MHz is so used for home/residential applications.