Bildschirmtext
From HOPE Wiki
Bildschirmtext was an early German online service. Technology wise it comes from the Teletex group of standards.
Control Codes
There is a good introduction into the control codes here.
https://github.com/bildschirmtext/bildschirmtext/blob/master/doc/CEPT-Codes%20f%C3%BCr%20BTX.md
Software
A software decoder can be found here: https://github.com/bildschirmtext/btx_decoder
The software on the central station is here: https://github.com/bildschirmtext/btx_decoder
The software modem for Asterisk is here: https://github.com/Casandro/btx_modem