Install libopal-dev by entering the following commands in the terminal:
sudo apt update sudo apt install libopal-dev
OPAL library header files
This package contains the headers and a static version of 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.