Why is "i" being used as the article "the" here?
It's just way we say it in Swedish:
answer the phone - svara i telefon
I svara i kommentar! See what I did there? took of the -en!
Then maybe we should recommend this thread which says something about how some words can be used in the indefinite in a sort of general sense: https://www.duolingo.com/comment/5824774
What :)? Did you think the "i" served as a determiner? Svara telefoni...
Maybe, because you can think of using the telephone as speaking "into" the telephone. And therefore you also answer "into" the telephone.
I have memorised it by using this tip, it really makes sense
can one say.... Han slutade att svara i telefonen?
Yes, if you mean a specific phone.
Kan jag saga
"Han slutade svara telefonen"
No, svara requires a preposition whenever transitive, and Swedish uses the indefinite.
why not telefonen
It's not really about a specific phone but about the act of answering, so Swedish prefers not to use the definite. Similar constructions make it more obvious - for instance: