In the context of future tenses, each option you've provided has a slightly different meaning or usage:
I'm answering - This is in the present continuous tense and is typically used to indicate that an action is currently happening or is a planned action that is already in progress. In this case, it implies you are in the process of answering the phone right now.
I'm going to answer - This expresses a future intention or decision that has already been made. It suggests that you have decided to answer the phone soon.
I will answer - This is in the simple future tense and indicates a willingness or determination to perform the action in the future. It can imply that you will answer the phone when you get the chance, without a prior decision.
For the scenario "The phone is ringing," the best option depends on the intent:
If you are already engaged in the action of answering, you would use "I'm answering."
If you have the intention to answer soon, "I'm going to answer" is appropriate.
If you are stating what you will do when possible, "I will answer" would be suitable.
Choose the option that best reflects your intention or action in the moment!