Both correct but here is the nuance:

• 没 + action: have not taken the action (yet)
• 不 + action: do not take the action (in a general/objective sense for telling the common sense, the laws of nature, theories, the decisions, etc.)

For example:

• 我很忙，所以没回你电话。I was very busy, so I did not (have the chance to) tell you. — the final result
• 我很忙，所以不回你电话。I was very busy, so I did not (want to) tell you. — the decision

有同的问题

Both fine, but 回电话 is much much more common. If you want to emphasize the action, then you can say 接电话 (answer the phone). 接 jiē means “to connect, to link”.

I don't think you can put 为什么 at the end like that.