I'm testing that the callback is called. I didn't know that tests must have an assert. In my experience, when this tests fails, it just times out because the callback is never called. What would be a more appropriate way to do this? Maybe something like this?

```
var onerrorStub = this.sinon.stub();

subject.init(button, null, function() {
  done();
}, null, 'outgoingCall');

subject._click();
this.sinon.assert.called(onerrorStub);
```