Translation:He was seen while selling my phone.
I moved to Vietnam so I have a lot of opportunities to get clarification of two year old problems here. One of the people I live with is a university professor of engineering. I got him hooked on English here.
Anyhow, best I could interpret our conversation on the phone vs telephone issue, he agrees with Duolingo on using phone and rejecting telephone. He says phone is thought of as just the device you use and telephone is the whole system of cables or towers and whatnot.