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.
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.