900 MHz is the better frequency range for passing through obstructions, like the walls of your house.
2.4 GHz is well into the microwave range of the spectrum. Microwave ovens use RF energy at about 2.5 GHz to cook food.
While the transmission power of a cordless phone is probably only about 1/4 watt (most newer consumer microwaves are 900-1200 watts), the idea of putting that antenna 1 inch away from my brain for a couple minutes to a couple hours at a time just doesn't sit well in there. Your brain is mostly water, and water absorbs microwaves.