There are cordless phones that work up to six miles or more, believe it or not. They work on a principle known as DSS or digital spread spectrum, as some high end cordless phones do, but with added power and an outside antenna. The most powerful model of cordless phone sold in the United States is by Engenius. Their phones feature an outside antenna which can be placed up on the roof or a tower.
Systems can function like walkie talkies with each other, as well as make phone calls, making the Engenius system ideal for construction sites, golf courses and other large areas. There are export cordless phones with work up to thirty miles, however these are not legal in the United States and use a frequency that will get you in trouble fast with both the FCC and ham radio operators. To find out more about legal high power cordless phones see: High Power, Long Range Cordless Phones