I’ve been using Danny’s extension
The RPi is connected to the Phantom using the toslink optical input - either directly on the Phantom or, if you have one, the Dialog.
You do need the Dialog to run 2 Phantoms in stereo, but the optical cable can still go into either of the Phantoms or into the Dialog - just need to configure it in Spark. I have it going into one of the Phantoms because of noise issues with the optical input on the Dialog that Devialet have not resolved yet.
The optical output is provided by the Hifiberry or IQAudio HAT on the RPi (running Roon Bridge) with the toslink optical output. It supports 24/192 and it works fine at that with the Phantoms - I’m using it now. Neither the HAT nor the Phantoms support anything higher.
The UPnP only comes into play to control the volume - the audio is not transported via UPnP.
Note - I only know this about the 1st gen Phantoms - which is what I have a pair of along with a Dialog. The later ones may support stereo with a Dialog.