This code will add a "PhoneNumber" custom editor for Backbone Forms. The formatting function was taken from: https://github.com/groupme/jquery-format-phone/blob/master/src/format_phone_input.js, and you could replace it as needed.

PhoneNumberEditor.js

Backbone.Form.editors.PhoneNumber = Backbone.Form.editors.Text.extend({
    initialize: function(options) {
      Backbone.Form.editors.Text.prototype.initialize.call(this, options);
      this.$el.on('keyup', function(e){
        var input = $(e.currentTarget);
        input.val(input.val().replace(/[^\d]/g, ''));
        var digits = input.val();
        
        if (e.which == 8 && digits.length <= 3) {
          return;
        }
        
        if (e.ctrlKey || e.shiftKey || e.metaKey) {
          return;
        }

        if (e.which == 8 && digits.length > 3) {
          input.val(digits.substring(0, digits.length - 1));
          input.trigger('keydown');
        }

        if (digits.length < 3) {
          return digits;
        }

        if ((digits.length >= 3 && digits.length <= 6)) {
          var prefix = digits.substring(0, 3);
          var rest = digits.substring(3, digits.length);
          return input.val(prefix + '-' + rest);
        }

        if (digits.length > 6 && digits.length <= 10) {
          var areaCode = digits.substring(0, 3);
          var prefix = digits.substring(3, 6);
          var rest = digits.substring(6, digits.length)
          return input.val('(' + areaCode + ') ' + prefix + '-' + rest);
        }
        return;
      });
    },

    getValue: function() {
      if(this.$el.val()){
        return this.$el.val().replace(/[\s-\(\)]/g, "");
      }
    },

    setValue: function(value){
      Backbone.Form.editors.Text.prototype.setValue.call(this, value);

      this.$el.trigger('keyup');
    }

  });