Internet cuts off whenever phone is used

When a DSL modem is connected directly to my computer, by ethernet cable, the Internet connection is stable, and I can use the telephone at the same time. But when I run the DSL modem to my Airport Extreme, and use the wireless connection, I cannot use both the Internet and the phone simultaneously. I can connect to the Internet, but this connection is cut off whenever I use the telephone.
When I make a phone call, my computer immediately loses the wireless connection with the Airport Extreme. It simply disappears from my menu bar. The moment I hang up the phone, it reappears, and I can use the Internet again.
I've checked with my service provider (both phone and Internet) and there is no problem on that end. Also, there's no problem if I connect the ethernet cable directly to my computer. It only happens when using the Airport Extreme.
I use a cordless telephone, a basic AT&T 2.4 ghz. Is it somehow on the same frequency as the Airport Extreme? Is there a setting in the Airport that can be adjusted to solve this problem? Thanks.