The name travels further than the credit
Because the brand is present in many markets, people assume credit is portable. It is not. Each national operation runs its own accounts and its own top-up system, and in some countries the brand is operated under licence by a different company entirely.
Get the country from the recipient, not from the logo.
Prepaid lines
Credit applies to prepaid accounts. Contract customers are billed monthly and cannot use it.
Terminology differs by country — "pay as you go", "prepaid", "pre-pago" — but the test is the same: do they top up, or do they get a bill?
Bundles and international calling
Most national operations sell monthly bundles, and several sell international calling or roaming add-ons. For a recipient with family in another country, credit spent on an international add-on is often worth considerably more to them than the same amount of general airtime.
Mention it rather than assuming they know.
Roaming
Where the recipient travels between countries, a prepaid line's roaming rules vary sharply by market and by bundle. Credit can usually buy a roaming pack where one exists, which is a genuinely useful gift for someone about to travel.
Amount
Whole bundles. Match the plan they use rather than picking a round number that lands between two plans.
Asking for one
Country, confirmation the line is prepaid, and the amount. See the mobile top-up guide and, on the general region problem, why region matters.