Install libpt-doc by entering the following commands in the terminal:
sudo apt update sudo apt install libpt-doc
Description:
Portable Tools Library documentation & sample files
This package contains the documentation and samples to program with PTLib, which is a moderately large class library that has its genesis many years ago as a method to produce applications to run on both Microsoft Windows and Unix X-Window systems.
Homepage: http://www.opalvoip.org/
Version: 2.10.11~dfsg-2.1
Section: universe/doc