As `MockBTAdapter` is a global object and may be used by other test files, we should always restore the spies and stubs. For convenience, it is suggested to use sinon sandbox.
```
// a global sandbox
var sandbox = sinon.sandbox.create();

// in the test
sandbox.spy(MockBTAdapter, 'answerWaitingCall');

// do this in teardown to restore everything created using the sandbox.
sandbox.restore();
```