Which is better? I want to know the difference.
- What is the best time to callback?
- When is the best time to callback?
Which is better? I want to know the difference.
- What is the best time to callback?
- When is the best time to callback?
Either, but I'm more concerned about callback. I can see no reason for making it one word.
Either will be understood, but "What is the best time to call back?" is more commonly used.
At a technical level, using "what" is modeling the answer as a generalized conceptual entity, while using "when" is specifically modeling it as a chronological time. Nobody is ever thinking about this distinction when using either phrase, however.