phoneField_1.js

var filter = [];

//since we're looking for phone numbers, we need
//to allow digits 0 - 9 (they can come from either
//the numeric keys or the numpad)
const keypadZero = 48;
const numpadZero = 96;

//add key codes for digits 0 - 9 into this filter
for(var i = 0; i <= 9; i++){
  filter.push(i + keypadZero);
  filter.push(i + numpadZero);  
}

//add other keys that might be needed for navigation
//or for editing the keyboard input
filter.push(8);     //backspace
filter.push(9);     //tab
filter.push(46);    //delete
filter.push(37);    //left arrow
filter.push(39);    //right arrow

/*******************************************************
  * onKeyDown(e)
  * when a key is pressed down, check if it is allowed
  * or not. If not allowed, prevent the key event
  * from propagating further
*******************************************************/
function onKeyDown(e){  
  if(filter.indexOf(e.keyCode) < 0){
    e.preventDefault();
    return false;
  }  
}