Go to PBX > Extensions > Ring Groups, click Add.

Configure the ring group.

Ring-Group Number : Use the default number or change the number.

Group Description: Give a name for the ring group to help you identify it.

Ring Strategy: ring all available agents until one answers (as default)

Ring Time: Time in seconds that the phones will ring, max 300 sec.

Extension List: Select the desired extensions to the list.

Destination if no answer: Define what will happen if none of the members in the ring group answer the call in the defined time.

Click Save and Apply.