Are all electrical sockets of my house in the same phase?
Here in North America, each house is fed from a single phase of the distribution system thru a step-down transformer.  The secondary of that transformer is 240 V center tapped.  All three lines go into your house.

The center tap is earth grounded near where it enters the house.  Ordinary 120 V circuits are between one of the ends and the center, which is ground.  High-power 240 V circuits, like for a range or dryer, are between both ends.

Therefore the hot side of one 120 V circuit will either be the same phase or 180&deg; out of phase with others.  It will also be the same phase as one side of the high power circuits, and 180&deg; out of phase with the other side of these high power circuits.