Prepaid, not postpaid
Credit goes onto a prepaid account. Customers on a monthly postpaid plan are billed differently and top-up credit generally does not reach that bill.
The wording people use varies — "pay as you go", "prepaid", "monthly plan" — so the reliable question is: do you top up, or do you get a bill?
Family and multi-line accounts
Where several lines sit under one account, credit added to the account may cover the whole thing rather than the specific line you had in mind. That is fine if it is a family gift and confusing if it was meant for one person.
If the gift is for a particular person on a shared account, say so and we will flag whether it can be targeted.
Network and country
One network, one country. Credit cannot be moved, refunded or converted. This is a US network; there is nothing to apply it to elsewhere.
Data is usually the point
For most prepaid users the monthly data allowance is what the credit really buys. Sizing the gift to a whole month of their usual plan is more useful than a round-looking number that lands between plans.
Who it suits
Students, people managing a budget deliberately, older relatives, anyone who would rather have something useful than something decorative. It is not a celebratory gift and it is frequently the most appreciated one.
Asking for one
Network, country, prepaid confirmation, and whether the line sits on a shared account. Why credit cannot cross from one network to another is explained in the mobile top-up guide.