'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.
The choice "twoj" was NOT even given??
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.
why isnt it possible to ask
Jaki twój numer telefonu?
Because it misses the verb ;)
yes, for sure - but in many sentences (even questions) there is no need for a verb in Polish. So can we form the rule, that questions starting with Jaki/a/e... require a verb?
Only sentences with a dummy pronoun (to) don't require a verb. However, I've observed that the form of "być" is sometimes dropped in colloquial speech. (Podlasie region)
thanks for the helpful comment. Indeed I was thinking, that być could be left out generally, but indicating that there is a need for to then, is a good thing to know.
Can this type of a question be asked with a "co jest" instead of jaki jest?
No, that wouldn't work.