cutting plaster wall

I have what I think is plaster walls. Where the trim is removed it looks like concrete. In one spot where I can see the back of the wall I don't see slats, but what looks like some kind of drywall.

Is this plaster on drywall? If so how hard is it to cut a square hole in it? I figure drywall would okay, but not sure on the plaster. I'd like to extend a single receptacle box to a double so I can fit in a phone jack/cable plug. Currently, the cables come up through the floor. I'd like to do the same thing in the other bedroom, it's got an old style four prong jack.