Translation:I will phone
I can never remember the rules for when it should be i, fi or mi (just whatever sounds natural!). But Gareth King has the following in "Colloquial Welsh":
Subject of a verb (south: sometimes fi): dw i, dylwn i, ga i (ga fi)
With compound prepositions: o (fy) mlaen i "in front of me", er (f)y mwyn i "for my sake"
After conjunctions and other miscellaneous words: ti a fi, pam fi?, pawb ond fi
After non-inflecting prepositions: gyda fi, â fi, heblaw fi "except me"
Object of an inflected verb: stopiodd yr heddlu fi, credwch fi
i and fi interchangeable
After inflected prepositions, but i is standard: arna (f)i, wrtho (f)i
As an echo pronoun to fy / as the object of verbnouns, with a preference for fi after a vowel: (fy) nheulu (f)i, (fy) nghlywed (f)i, (fy) ngharu (f)i
North: in i mi "to/for me"
North: in ddaru mi "I did" (alternative past tense construction)
Those rules will stand you in good stead. And as @ibisc says, context is huge - something which is hard to replicate on a course like Duolingo.