I did something like this for a 900 MHz cordless phone. I had one inside my cubicle at work in my "flipper". Since it was surrounded by metal on 5 of 6 sides, I didn't get very much range. I simply wrapped wire around the antenna of the base and ran it outside of my "flipper" with about 3 or 4 inches sticking out. I could use my cordless phone anywhere on our large manufacturing floor.
Remember, most cell phones operate around 1900 MHz. You can't use just plain old twisted pair wire or television coax if you plan on having any length to your feedline. Have you considered buying a suction-cup antenna they sell for automobile windshields and sticking that to a window and connecting your phone to it? It wouldn't allow you to walk around your house with your cell phone, but it'd be much simpler.
Something else to consider. Do you get any sort of signal outside of your house? I switched providers recently and noticed I have very low signal in my house. I suspected something like foil-lined insulation, but even outside of my house I have no signal. A passive repeater wouldn't help.
John Mayson Austin, Texas, USA