The Portland station that does it is KBOO an independent locally funded community radio station. They were usually #2 on my dial before I had to get up at 5am every morning. Their morning programming is chat and hideous shit like traditional Irish or American Indian music which doesn't go with the shops flow so I don't listen all that often. They did either a half or full hour prison show with messages.
I listen to a different locally funded radio station now, only on weekends. i know a few of the DJ's and it's a better music mix. https://xray.fm/
The Detroit show was just the later night DJ on a contemporary urban format station who would read out messages to prisoners.
Many of the messages are hopeful, many are sad they are filled with a lot of love. Nothing like a shout out that a favorite uncle died and we love you and are waiting for you to get out.