Sure, mobile or mobile phone are also accepted here. The American terms are always the suggested ones, as Duolingo is American, but we've added UK terms whenever possible.
(Although I fail to see what stops someone from typing in cell phone on Duolingo just because they wouldn't use the term in their everyday lives.)
After "minulla / sinulla / hänellä / meillä / teillä / heillä on ikävä = to miss" you have to use partitive case.
I miss the cat. = Minulla on ikävä kissaa.
Now we have "that cell phone" which in Finnish is "tuo kännykkä" in nominative case. Of course in order to form the sentence in a correct manner we have to put this into partitive case which is "tuota kännykkää".
It is the partitive case. Here is a link that explains it: https://uusikielemme.fi/finnish-grammar/finnish-cases/grammatical-cases/the-partitive-case-partitiivi