One of the least expensive ways to make a cell phone signal booster is to take a twist tie or paper clip and place it into the port for an external antenna, or duct tape the paper clip or twist tie to the phone. Some people have made a signal booster using antenna parts and cans.
Another method to try is putting a drinking glass that does not have any liquid in it over the phone. If this is the method used, make sure to use the speaker function or a good headset. If these methods do not work, try getting an external antenna if the phone has a port for one. Something to try if unable to get an antenna is to look into the more expensive methods of using femtocells or repeaters, which most cell phone carriers will sell to the consumer.