"Hello? Are you Doctor Li?"
That's exactly it. Duolingo wants the Chinese punctuation. (For some questions you can also get away with no punctuation at all, but I'm not sure this is consistent throughout the course.)
With my keyboard when I'm using the Chinese input I can change the punctuation style with [Ctrl+.] ("control period"), but it may depend on your settings.
2020.6.4 Now that you mentioned it, I agree it should be wei2 for answering the phone.
Great that you spotted that
I checked for examples in Plexo, and they have some audio with wei4 for answering the phones too
Guess, I'll have to wait for some drama scenes to see how it's naturally pronounced. lol
Maybe wei4 is the pissed off way of saying hello on the phone
at first I did it without spaces but it marked that as wrong and I thought it might of been because I didn't put any punctuation, used punctuation but still didn't get accepted, only thing I could see was different was the spacing because for some odd reason or another my chinese keyboard that would automatically space after punctuation doesn't do it anymore so I tried to do it manually, copying exactly what they have as the correct answer.