PBX usually involves multiple telephones sharing the access to a smaller number of incoming lines. With one line coming in a PBX seems a little overkill. (IMO) If he
was looking for internal features such as speed dial, line busy indicator, intercom, baby monitor, etc - I think the wireless system such as V-tech below might be more efficient:
Independent of any solution, the cables are still waiting to be terminated. The BIX block is basically a patch panel that terminates the cable on the back side. The BIX is mounted and each cable termination can be accessed from the front with a jumper cable. Basically you are simply bridging the Reds, Greens, Yellow & Black all together so you may find a screw terminal much easier to use.
My two cents.