Edit in JSFiddle

var contact = new Backbone.Model({firstName:'John',lastName:'Doe'});

contact.validate = function(attributes,options){
    var validPhone = /\(?\d{3}\W?\s?\d{3}\W?\d{4}/.test(attributes.phone); 
    if(!validPhone){return 'Setting or Saving Invalid Phone Number Attempted';}
};

//Have Backbone listen for invalid event on contact model
Backbone.listenTo(contact,'invalid',function(model,error,options){
    console.log(model,error,options);
});

//setting invalid phone number will trigger the invalid event
contact.set('phone','111-1-1111',{validate:true}); //will not set, fails validation