"Hello? Are you Doctor Li?"
Yeah, but when you are strengthening skills it isn't necessarily clear which lessons they are drawing from.
因為在電話中聽畫者只能是"你"，這個"你"常常會被省略。 the one listening the phone can only be "you," so the word "you" is often omitted.
I think if you frame the sentence this way, then you don't add the 吗 at the end. It's either you add 吗 at the end, or you do the verb+不+verb thing when asking a question. Not both.