This just doesn't feel right...

idk.js

var app = app || {};
app.filter('prettyPhone', function () {
  return function (input) {
    input = input || null;

    if (input) {

      if ((input[0] == '(') || (!~input.indexOf('-'))) {
        return input;
      } else {
        var area = [],
            three = [],
            four = [];

        for (var i=0;i<input.length;++i) {
          if (i <= 2) {
            area.push(input[i]);
            continue;
          }
          if (i <= 5) {
            three.push(input[i]);
            continue;

          }
          if (i <= 9) {
            four.push(input[i]);
            continue;
          }
        }
        var pretty = '(' +
                  area.join('') +
                  ') ' +
                  three.join('') +
                  '-' +
                  four.join('');
        return pretty;
      }
    }
    return input;
  }
});