Why can't I use the function input as a variable to call the function?

Question

How do I get this function to put the value for num as the call to phrase. ? I specified 1 as the input value, so why doesn't it read as phrase.1[0] ?

var phrase = {
    1: "Breaking News! The world in crisis!"
    };
function writeText(num){
    var dialogue= phrase.num[0];
    dialogue= dialogue+ phrase.num[1];
    document.getElementById("dbox").innerHTML = dialogue; 

};
writeText(1);

or even this?

var phrase = {
    first: "Breaking News! The world in crisis!"
    };
function writeText(whichPhrase){
    document.getElementById("dbox").innerHTML = phrase.whichPhrase; 
};
writeText(first);