Install asterisk-moh-opsound-wav by entering the following commands in the terminal:
sudo apt update sudo apt install asterisk-moh-opsound-wav
asterisk extra sound files - English/wav
Asterisk is an Open Source PBX and telephony toolkit. It is, in a sense, middleware between Internet and telephony channels on the bottom, and Internet and telephony applications at the top. . Asterisk includes a set of standard sound files in various formats. This package contains several "on-hold" music files WAV format (8Khz, mono).