My old nokia is still working well
I think matkapuhelin should also be accepted.
"I have a good mobile phone" should also be accepted for kännykkä. The name for these devices varies a lot within the English-speaking world.
Why is hyvin not acceptable when referring to the mobile phone?
Hyvin means well, it's an adverb.
What's a difference between hyvä and kiltti?
when do you use kiltti rarher than hyva
Kiltti refers to people, for example kind, gentle, nice.
Hyvä means good, and its use doesn't usually overlap with kiltti.