Just an observation - where I live, the distinction in speech between a mobile phone and the ordinary type is dying out. Hardly anyone says "cell phone" or "mobile phone", and if they say "smartphone" they're not referring to the device in its making-phone-calls capacity. So just "phone" shouldn't NECESSARILY be accepted here, as this could be just a dialect thing, but that's what I'd naturally use in speech to express the concept.
Because of the lenition afterwards. a fon póca would be 'her'
a fhón -> His phone (lenition)
a fón -> Her phone (nothing with consonant)
a bhfón -> Their phone (eclipse)
In the browser, there’s a speaker icon for this exercise (which I presume means that there’s a recording available for it) — does it actually have a recording of a different phrase?
You have question marks, but it's not obvious what your question is. Are you asking why that answer was accepted or asking why it wasn't accepted?
"his mobile phone is at me" is a very literal translation of "tá a fhón póca agam". In fact, it's an overly literal translation - when someone says "Tá X agam" in Irish, they almost always mean mean "I have X", and Duolingo would be doing you an injustice to suggest that "his mobile phone is at me" is an acceptable translation of "tá a fhón póca agam".