So currently I have 3 phone sockets in the house, with 2 phones, one each on socket A and B. When someone calls, both phone rings and I can pick up either. I want to replicate this when I switch to naked DSL+VoIP.

The house is old so there's no demarc. Master filter is installed at socket A. There are three sets of wires inside socket A.
First set is a black and a white wire. They are connected to the orange and white wire of the master filter.
Second set is a cable containing 6 wires. Two wires are connected to the blue and white wire of the master filter. Two wires are connected to socket A. Two wires are not connected.
Third set is also a cable containing 6 wires. All six are connected to socket A.

Finally, the DSL modem socket is connected to the green and white wires of the filter.

I plan to place the ATA at socket A.

So can I plug both the ATA and a phone into socket A using a double adaptor, therefore making socket B connected to it too? Which wires should I cut to disconnect the outside line?