I think it's more readable if you do:
```javascript
SilentSms.checkSilentModeFor(1234).then(function(isSilent) {
  assert.isTrue(isSilent);
}).then(SilentSms.checkSilentModeFor.bind(null, 999))
.then(function(isSilent) {
  assert.isFalse(isSilent);
}).then(done, done);
```