Well, on paper (or screen ) it looks fine. Given that the problems occur when you have the switch in circuit, and you don't (as I read it) have to throw the switch to cause issues, I can only guess that the switch circuitry is the issue. It appears to be a US unit, so could it be something to do with that being incompatible with BT circuits?

There's never a legend about when you need one...

try putting the two filters in series on the BT connection so that the line to the switch is double filtered and the Livebox is fed its ADSL from the first filter.

If that doesn't work try a microfilter on the Livebox VoIP output.

Thanks fellas.

I'd already tried the double filtering idea. To no avail.

I've just tried it with a filter on the VoIP output, but that still causes problems.

There is a bit of blurb on the ebay listing about needing to use the correct RJ11>BT adaptors in order to get the correct pins connected. I've got some decent modem cables and RJ11 adapters, so I think that shouldn't be a problem.

How can it (as it appears to be) 'go through' the filter and cause problems with the ADSL data? Is that possible?

the design of the livebox may be such that noise on the VoIP port can affect the modem, or maybe the switch itself resonates at ADSL frequencies or something. An oscilloscope or spectrum analyser would answer the question.

Be simpler if they just integrated the PSTN line that provides the ADSL into the VoIP component to connect to a single phone.