The appropriate answer depends on the context. Here are some possible responses:
If you are asked to provide your own phone number:
* "Mi número de teléfono es [your phone number]." (My phone number is [your phone number].)
If you are asking for someone else's phone number:
* "¿Puedes darme tu número de teléfono?" (Can you give me your phone number?)
* "¿Cuál es tu número de teléfono?" (What is your phone number?)
If you don't want to provide your phone number:
* "Lo siento, no puedo darte mi número de teléfono." (I'm sorry, I can't give you my phone number.)
If you are unsure who is asking:
* "¿Quién pregunta?" (Who is asking?)
* "¿De quién es este número?" (Whose number is this?)