you can use:
```javascript
sinon.assert.notCalled(ContactRenderer.prototype.render);
sinon.assert.calledWith(Template.prototype.interpolate, {
  number: participants[0].number
});
```
This gives better errors when it fails.

See http://sinonjs.org/docs/#assertions for more assertions. I especially like also `calledWithMatch` when an object is passed to a function and you want to test only one of the properties.