See: https://www.openimsd.de/10-ringback-tone.html
4G phone call / VoLTE / packet-switched call
There are no traffic channel to save, but there is bandwidth, even a VoLTE calls uses only a little bit of bandwidth. VoLTE is based on SIP (Session Initiation Protocol). For SIP calls, the default case is the ringback tone generation is done on the phone. There is a special case, called early media, where the network will generate the ringback tone, but usually this is only used when giving you a custom ringback tone.
Qualcomm basebands
Back to OpenIMSd. When I started to do VoLTE calls with the Pixel 3a or Oneplus 6T, there were no ringback tone.
Qualcomm basebands has a special TLV for ringback tone generation in the voice call status messages, I’ve not yet tested the exact behaviour with 2G networks yet, but for 4G network, the baseband adds the TLV and tells the user space to generate the ringback tone.
Is this also relevant for the Librem 5?