Same name, different companies
The brand appears in several countries, and those operations are distinct businesses with distinct systems. A code bought for one will not top up a line on another.
This catches people with family abroad especially often, precisely because the name is familiar and looks international. Confirm the country, not just the network name.
Prepaid lines only
Credit applies to prepaid accounts. Contract customers are billed by direct debit and cannot use it.
If you are not sure which the recipient has, ask them whether they top up or get a monthly bill. That is a question anyone can answer.
Plans and add-ons
Prepaid customers typically choose a monthly plan from the credit on the account, and may add data packs on top. A gift sized to a whole plan is cleaner than an arbitrary figure.
Where the network sells international calling add-ons, credit covers those too — worth mentioning for a recipient who calls family abroad, because that is often exactly why they are on prepaid.
eSIM and travel packs
Some networks now sell data-only packs for travel. Where those exist, credit can usually buy them, which turns a top-up into a genuinely useful gift for someone about to go abroad.
Amount
Whole plan periods. One month, three months, six.
Asking for one
Network, country, and confirmation that the line is prepaid. Context in the mobile top-up guide.