That doesn't necessarily mean that you must retire your cordless phone if you install a WLAN. 2.4 GHz cordless phones based on DSSS can often coexist with 802.11b WLANs. Results vary by phone manufacturer, so if you run into trouble, borrow someone else's phone and try it. In my home office, I use 2.4 GHz cordless phones and they do not unduly interfere with my 802.11b WLAN.
Common sources of interference with 802.11b (and now 802.11g) WLANs include 2.4 GHz phones, Bluetooth devices, older/leaky microwave ovens, neighboring WLANs, and other kinds of radios using the unlicensed ISM band. WLANs in locations with excessive ISM band interference can move to 802.11a. The 5 GHz UNII band occupied by 802.11a is much less crowded, but there are other radio applications in that band as well.