Why is sound coming from the earpiece instead of the speaker in iPhone?

ok from what i understand when you receive a phone call the behavior of the iPhone is to make the sound be generated from the ear speaker at a low volume to be heard on the ear until you choose to go to the loudspeaker, and in the iPhone since the iPhone 7 if I'm not mistaken you do have two speakers for that, one near the charging port and the second is the same on-ear speaker that is turned to a loudspeaker and mainly calls on the loudspeaker are received on this one.

if a headset is connected the default behavior is for the ios device to output audio using the headset.

is this a regular phone call or an app-based call like whatsapp?