That is called "Wireless Home Intercom System". I think The Best Wireless Home Intercom System is "RWIS - Chamberlain Reporter Wireless Home Intercom Kit Set".
Stop guys. Mobiles are becoming more cheaper. You get a brand new handset for just Rs. 750/- or even cheaper. And then go for a double SIMM card package (most operators offer two connections pack). You can communicate over both the connections for free and even stay in touch no matter which part of the city you are. Cordless phones are restricted to some extend but are also a cheaper solution.
Voyager CL-328XP - 1 to 10km range. Yes, that's 10km. http://www.voyagerphones.com/CL-328SX.html Without knowing your budget or location, nobody can help you. There are plenty of seriously long range cordless phones around. Use Google.