First try the free Data Interface app ,see if there is some
output on your screen,and maybe you are interested in this Pro app

Update to the latest Google guidelines Android 13.

- runtime permission record audio
You will be asked for permission to record audio according to the latest Google guidelines, the user must give runtime permission if the audio jack is used. (Allow record audio.)

If the audio jack is used to decode Pocsag then Android sees this
as Record Audio but nothing is stored on the phone ,it is decoded.

This app has the following improvements or changes compered to
the free Data Interface app when decoding POCSAG.

- improved audio-level detection
- improved trigger level detection
- POCSAG data can be heard through speaker.(>=4.2.1.0)
- simple address filtering
- email filtering results
- email messages 60-720 minutes
- bit triggering manual(0-80)
- night mode
- read / write settings

The audio jack is one of the interfaces on mobile devices
and tablets, it's mostly used to play music ,but it can also
be used to receive and decode data.

It can not be connected directly to a receiver, this can damage the receiver/scanner,so a small circuit is needed.

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POCSAG is an asynchronous protocol used to transmit data
to pagers. Before the smartphone pagers were very popular
Small text messages could be sent very cheaply.

Today the protocol is still used in many countries.
It is the dominant radiopaging technology in Fire Departments,
Rescue,Medical Paging,industrial signalling
hospitals, hotels and other facilities.

Pocsag alerting networks are physically independent of public
networks or Internet connections so it is a robust alerting network.