Install libopal-doc by entering the following commands in the terminal:
sudo apt update sudo apt install libopal-doc
OPAL library documentation files
This package contains the documentation and samples to program with the OPAL library. . The OPAL project aims to create a full featured, interoperable, Open Source implementation of the H.323 and SIP protocols that can be used freely by everybody. These protocols are most used for Voice over IP (VoIP) conferencing.