Doesn't sound that hard actually...
Buy a Mini Box (the smallest they have will work, it is like 1"x3"x1/2", get a metal switch, get some intercom wire and 2 line extenders (those things that you use to make 2 lines into one long line).
now, your metal switch (kinda like the switch for your lights only for electronics, it will have 6 points (3 sets of 2, first set will be line 1, second set will be to the phone, and the last set will be the second line).
Now, split your line extenders in half (so you have two parts) and cut the wires. Now cut about 3 2" lengths of wire.
One one of the lengths of your intercom wire (two actual wires per length) you need to seperate it about 1/4th an inch and solder red/black to your switch. Do this for each wire length until all three of your lengths are wired to your switch.
Now with your line splitters solder the red wire in your line splitter to a black line on your intercom wire, and green from your line splitter to white on your switch.
When you finnish this you'll have three sets of adapters coming from yoru switch, now apply the switch to your "mini box" or whatever, you could use lego's for all i care. now you will need to stick the center wired adapter to the front of your box and the two remainders to the end section of your box.
You should now see two inputs (on bottom) and one output (on top).
Plug line 1 and 2 into the inputs, and your telephone to the output.
It SHOULD work fine, not too hard to do, most expencive thing would be the mini box, most of the other stuff I just had laying around the house.