Whatever the person is hearing is not included at all (unless it is on speakerphone), and whatever the person on the phone says is put down just like any other utterance, in quotes, not italicized, separate paragraphs, etc.
If the POV character CAN hear the other side of the conversation, say on a speaker phone or just because it is loud enough, you have a choice, you can also do that in quotes, or you can do it in italics to indicate it is on a device.
This text you sent. I don't believe that, Jonathan heard. It was tinny, he did not recognize the voice.
Mark said, "Whatever, dude. That's what I heard."
Jonathan wondered what the text was.