Kocrachon:You COULD buy one of those sticks that you stick on the back of the phone thats like 10 bucks at best buy or anything that sells the "Wireless" brand stuff. But I doubt they work...
those stickers don't work.
If you don't want to spend the money for a cellular repeater, the best thing you can do is to buy a bluetooth headset or speakerphone, then set the phone in a part of the house that gets better reception and is within 25 feet of the dead zone (on the first floor, directly above the place in question) then rely on the bluetooth.