"Last month, I called my friend in Japan over the phone."
i find this a little wrong, its saying 日本の友だち implying that the friend is infact japanese and not just a friend in Japan. This needs to be worded better to say 先週、おれはともだちが日本に電話をかけました。this follows the english translation much more closely cause we dont know if the friend is actually Japanese or not.
の connects two nouns. This can often be translated as a genitive ('s), but other times a compound noun or even some kind of adjectival construction is better. In this case, 日本の友だち can be seen as "Japan-friend", which can mean either a friend who is from Japan ( = Japanese) or a friend who is in Japan, which is what we want.
を is the direct object marker. 電話 means telephone, かけました can mean many things but in this case translates best as "made"; together, 電話をかけました means "made a call (by telephone)".
に has a few different uses, but in this case it works as an indirect object marker, showing who you made the call to.
I used the kanji for かける in my comment (currently a short way up the page from yours) where I wrote what the full kanji version of the sentence would be. ^^
Can check out the following link to see which kanji is used for the word, especially what it says under the "etymology" heading:
However, it's not common to write the word with the kanji. The usual way it's written is the same as how Duolingo wrote it, with 電話 in kanji but かけました completely in hiragana. ^^
I was messing around doing the placement test on a different account just now. I avoided answering with kanji to try to stop it marking my answers incorrectly... Yet it still rejected this, even though it's the exact same as the default answer only written fully in hiragana:
The correction it showed was just the default answer, without telling me which part of my sentence it didn't like:
The default answer is only half in kanji. The full kanji version would be this:
I like how it expects people to psychically know the exact level of kanji it wants. Usually it accepts any sentence written fully in hiragana, but apparently not this time. XD
It could have corrected you for “にっぽん”. Although that is correct, try again using “にほん“。I did not even really know about “nippon” until I had a conversation with an elder person (I was studying in Tokyo at the time) and he said it’s mostly the older generation that still says it like that, which is traced back to before WWII.