Further to my previous reply . . . amazing how when you figure out the right search key words, Google gives the answer. In this case plug detection is achieved like this . . .
''USB Type-C connectors have 4 pins for ground, and 4 pins for VBUS. By specifications, all similar pins must be connected together on PCB. Same requirement is for Type-C plug, VBUS pins and GND groups must be tied together when mounted onto PCB".
“However, it is possible to free one of pins (in VBUS or GND group) on mainboard and use it to check whether it is tied to the rest of pins or not. When “mounted plug” (not “raw plug”) is plugged in, the host will be able to detect this.”
In Phonak’s case, they will have used non-standard cable wiring but using the same trick (I think). With the right chip, plug detection can be achieved with standard cables. So I’ll need to get me a cheap eBay Roger to mess with. Meanwhile I’ll use the Roger cable and a kluge of cables, adapters and power supply. Nothing is simple . . .