Very easy, pick up a radio scanner, it all depends on what frequency they operate on. Great thing with cordless phones is you can capture both sides of the conversation and not one side. If they are using spread spectrum which a lot do, then its gonna be much more difficult.
Here's a site where you can buy some: http://www.radioworld.co.uk
I would like to cite the legal aspects of this:
The legality of radio scanners varies considerably from place to place. In the United States it is a federal crime to monitor cellular phone calls. Some US states prohibit the use of a scanner in an automobile. Although scanners capable of following trunked radio systems and demodulating some digital radio systems such as APCO Project 25 are available, decryption-capable scanners would be a violation of United States law and possibly laws of other countries.
A law passed by the Congress of the United States, under the pressure from cellular telephone interests, prohibited scanners sold after a certain date from receiving frequencies allocated to the Cellular Radio Service. The law was later amended to make it illegal to modify radios to receive those frequencies, and also to sell radios that could be easily modified to do so. This law remains in effect even though few cellular subscribers still use analog technology. There are Canadian and European unblocked versions available, however these are illegal to import into the U.S. Frequencies used by early cordless phones at 43.720--44.480, 46.610--46.930 MHz and 902.000 – 906.000 MHz can be picked up by many scanners. The proliferation of scanners led most cordless phone manufacturers to produce cordless handsets operating on a more secure 2.4 GHz system using spread-spectrum technology. Certain states in the U.S., such as New York and Florida, prohibit the use of scanners in a vehicle unless the operator has a radio license issued from the FCC (Amateur Radio, etc)  or the operator's job requires the use of a scanner in a vehicle (ie, Police, Fire, Utilities)