Just break into their apartment and run a long phone cord in to their closet, build a little fort out of the shirts and stuff for extra covert. When they use the phone, just pick up yours (use a beige phone, they are better for this) and listen like crazy. When you find the information you need, casually walk out of the closet and murmur audibly about getting lost on 5th Avenue.
Yeah, your idea is really impractical. As z09 said, you'd actually have to rip walls apart to do it. Besides the fact that ripping walls apart is very hard to do in secret (without an army of starving termites at your disposal), how do you plan to find which line goes to your mark's apartment? It would actually be easier to build them a new apartment, and tap the line before you put up the gyprock.
Just box the TNI. Sheesh.