When extension is on the call (busy or in-use) caller should hear a different tone than ringing tone. In that case, call waiting tone can be used.
On extension's incoming/outgoing dial options r(callwaiting) needs to be added.
For example, incoming dial options would look like this: tr(callwaiting).
So when extension's state is busy or in-use, system will play a call waiting indication (from indications.conf) file.
Additionally, this "sound file" or more precisely tone cannot be replaced with some uploaded sound file.
Basically, system is reading frequencies from indications.conf file (for one country that is selected under System ---> Servers/Tenants ---> Locality ---> Indications).
More information can be found on the link:
https://www.itu.int/ITU-T/inr/forms/files/tones-0203.pdf