When A wants to know the telephone number of B, do we say:
A wants to ask B about B's cellphone number
A wants to ask B for B's cellphone number.
Asking about something refers to a topic, for example, you would ask a coworker about their weekend. You can't actually get someone's weekend activities, as it is not an object, but you can learn more about what they did, where they went, etc.
Asking for something refers to an actual object, for example, you would ask for a hamburger at a fast food restaurant. You can ask about a hamburger, but you would be asking about the details of it, like the ingredients, if it is organic, etc.
For your specific sentence, I would use for instead of about. You are asking for an object that you will be using, not details about the number.