Wi-fi certification by the Wi-Fi alliance encompasses 802.11a, b and g. As a catch phrase most people recognize 802.11g because that's what they know but .11a also falls under this umbrella.
But if like most you have .11b or g gear then many 2.4ghz cordless phones can have an impact on performance or connectivity in total. But not all. Why is that? Because there are direct sequence spread spectrum phones on the market. Not very common. Most 2.4ghz cordless phones use Frequency hopping spread sprectrum and Jack is right about those. Little chance if the FHSS cell overlaps your .11b or g WLAN cell that the WLAN equipment won't be impacted at all. how much depends on alot of variables. So as a generalization you could say if you get a FHSS phone and have 2.4Ghz wireless LAn equiipment, the chance of there being a problem is very high.
As a rule of thumb, if you have 2.4ghz goodies for your wireless network, getting a 900Mhz (what I prefer) or 5.8 ghz phones is the best option.
There is also the little know fact that most 802.11a LAN equipment uses the lower 5.3Ghz band while 5.8Ghz is typically used for WAN bridging. Been awhile so if I am crossed on that someone correct me. Could be the other way around but all the 5.8Ghz gear I've ever used, or mostly used anyway, was implementing point to point or point to multipoint bridging. So typically even if you have 802.11a LAN equipment, a 5.8Ghz phone would not affect the WLAN at all.
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