What is 您的第一個電話號碼是什麼 in English?

The literal translation of "您的第一個電話號碼是什麼" is "What is your first phone number?".

However, depending on the context, a more natural English phrase could be:

* What was your first phone number? (If you're asking about a phone number from the past)

* What's your first phone number? (If you're asking about someone's initial phone number, perhaps when they were younger)