DSL, LAN down when use phone

Hi, I'm using SBC as provider, and Dlink 604 router (wired router) for my network. I have a phone AT&T 2.4gb cordless phone. I set up like this

Plug
|
|- dsl line to router
|- 2.4gb phone
|- 900mhz phone

before, when I use the 900mhz, it is ok (when I'm talking, dsl never disconnected). But when I plugin one more phone (the 2.4g phone), whenever I use it, the dsl router disconnects (I can't browse any more) until I off the phone.

What's the problem? how to fix it?

 

 

 

 

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