Where can I get these ?

[Channel Hopper]

were the RJ10 straight through and wires going to same place on both sides of the RJ10's
Look at the wiring on the Duet cable and then the BT plug, compare that with the adaptor and then you can figure out what RJ10 - RJ10 you need.

I don't require a RJ10 - RJ10, its either an adapter for the 431A end, or a suitable Glensound replacement cable sporting RJ10 and RJ11 ends.

 

[RustySpoons]

I don't require a RJ10 - RJ10, its either an adapter for the 431A end, or a suitable Glensound replacement cable sporting RJ10 and RJ11 ends.

But RJ10 will work in RJ11 and will be off the shelf...

 

[Channel Hopper]

Whilst it does fit into the RJ11 socket, is it a true compatibility connection-wise (like the RJ11 into a RJ45 socket) ?

I've tried the one(s) here btw, though they have four cables inside, and the TBU does not pass through the signal to the Duet.

 

[RustySpoons]

Whilst it does fit into the RJ11 socket, is it a true compatibility connection-wise (like the RJ11 into a RJ45 socket) ?

I've tried the one(s) here btw, though they have four cables inside, and the TBU does not pass through the signal to the Duet.

That's probably because there is a pin miss-match somewhere, get the multimeter out
They are all the same but less pins, RJ45 will take any. You do get some odd ones where the retention clip is offset but they are custom ones and designed to stop people plugging in to wrong thing.