I think what I described in #4 will block calls to the phone and the answeing machine if that's what you want.
I've taken a look at the manual for the CPR V.106 call blocker
http://www.exteradirect.co.uk/downlo...-106-model.pdf
and I think I can see a problem implementing it though.
If the blocker as sold in the UK really only comes with a short RJ11-RJ11 lead to connect it to your phone, then you will need to improvise something to share it with a second device.
Have you got a BT style double adaptor and a spare ADSL/VDSL filter?
If so try connecting them like this:
If it doesn't work try different permutations of LINE 1&2 and TEL 1&2 as it says in the manual.
If I've misunderstood your problem then ask again, but try and give us a bit more detail.