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
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:
Why is "han stoppade svara telefonen" incorrect,?
We don't use stoppa with verbs, and it would have used an att. It's also idiomatic to use i telefon rather than telefonen.
where does the 'i' come from?
han slutade att svara mobilen why is it wrong?