Friend bought Panasonic KX-TGA 431CB Cordless phone (2).which is 5.8 mhz.
Has Sympatico W2 gateway Modem/router. I think that its 802.11/g which should be 2.4 mhz.
The phone's RF signal interferes with the modem. Cuts out modem if he is on phone line, speaking?
So he moved the modem 3 ft away, still gets interference.
To put a damper on this one, he is retired from Motorola, used to install towers, etc.. for mobile communications.

He will try to move modem with a 15 ft. RJ45, But doubtful that is the answer.

Phone master is in kitchen 24 ft away. phone used with computer is only with charging base.
Would the master require a filter?