"What is your telephone number?"
Translation:Jaki jest twój numer telefonu?
'Your' could also be 'twój'. Polacy, z którymi rozmawiam, mówią "Jaki masz numer telefonu", bez 'jest'.
Yes, you are right, that is the natural way to say it. I think they wanted word by word translation, because this is easier to learn. And the comment section is exactly for comments like yours - that give another option.
This used to star plural "wasz" (which is possible, but less likely) and the formal forms, not only "twój" will be the starred answer, so it will definitely be there.
how come you can write "jaki masz numer" and it's accepted but if you write "jaki jest twój numer" it's not accepted without "telefonu"
Actually, I'd put it the other way round... "Jaki jest twój numer" is almost certainly about your phone number (added), but "Jaki masz numer?" wouldn't strike me as a question about a phone number... I'd expect it to be a number in a queue or sth like that.