If it is important to the operation of the function, from an outsider perspective, the intermediate call should be tested. Oftentimes this isn't the case, though. Most often, I see intermediate calls spied/mocked when they have side effects to be avoided. This is actually a sign of tight coupling between modules, and patterns like dependency injection can help make it easier to test.

The trick for me is focusing on what the unit does from a consumers perspective. Avoid testing implementation details (unless they are important side effects), and test the behavior that does not change. If you do that, then refactoring becomes easier, because tests will only break when the contract of the unit changes.