Can anyone explain me why the verb occupies the third position in this sentence?
Agreed, pekarekr. Your sentence is is semantically and grammatically equivalent to the Duo translation that is labelled acceptable.
The correct way we would form this sort of question in English is "IS" + "VERB (in present continuous)." There seems to be no reason why "Is he perhaps/maybe talking on the phone?" should not be accepted! Another suggestion below, "Is he perhaps on the phone?" (from SusanKreid), should also be accepted.
What about, "Is he perhaps on the telephone?" It was marked incorrect, but is a more natural word order for me. Am I missing a nuance in the Swedish word order?
The above possibly highlights one of the difficulties with Duolingo, and that is knowing what English translation is acceptable. In this instance "perhaps" and "maybe" have the same meaning in English ie "it is possible that". Your prefered option of "maybe on the phone" is the most usual way it would be said in England, but surely any combination of "perhaps" and "maybe" together with "talking" or "on" the phone should be accepted.
Doesn't the English translation "Maybe he is on the phone" mean that the speaker is in a state of uncertainty about what the person talked about is doing, maybe he's on the phone, maybe he's in a delusional state and talking to himself. Whatever, isn't this declarative, not a question. Maybe he is on the phone.
that is what I thought it meant, but pratar threw me off. I remind myself that if I am confused by odd Swedish word order Swedes learning English will be just as confused. :-)