Interference from 5.8 GHz cordless phones

I have a 2Wire 2701HG-B DSL Modem/Router connected to AT&T DSL. I have a wireless laptop, two cabled desktops and a wireless xbox connected to it. Whenever I call out on one of our 5.8 ghz cordless phones the internet light on the modem/router goes red and it drops the connection then reconnects. When I hang up the cordless phone it happens again. The central unit for the cordless phones is connected to a DSL filter on its own telephone jack in a different room from the router. It doesn't happen when I call out using a corded phone. How can I tell what frequency the network is using? Can I do something to prevent this interference?