Install asterisk-moh-opsound-gsm by entering the following commands in the terminal:
sudo apt update sudo apt install asterisk-moh-opsound-gsm
asterisk extra sound files - English/gsm
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 in raw gsm-fr format (Compressed. Takes relatively little space. playable with sox).