It's a terminology thing.
In the scriptwriters world it is a voice over, because in the screenplay the person on the phone is not seen.
In audio post a narrator is a disembodied voice talking over the visuals. 99% of commercials have a voice over the visuals, everything from "This Friday, at...." to the phone number and sped up disclaimer at the end - in addition to movie narration, which is a voice over. So in audio post any voice not associated with a specific person speaking in sync with the visuals on the screen is a voice over.
In a phone conversation in a film it's just that - a dialog between two people, so even though the person on the phone is unseen it is actual dialog, so is a part of the dialog tracks. The phone dialog may be put on or sent to a "Futz" track (where we futz around with the sound - phone effects, special effects, you know, the fun stuff), but it's still a part of the dialog stem.
So both interpretations are correct; one applies to scripts, the other to audio post.
It drove me a little crazy when I started reading scripts, so ingrained was the audio interpretation of the "voice over" thing; it took a while until I got used to it.