Would you tell me whether "listen to" entails "hear"?-- No. 'Listen to' is a willful act, while 'hear' is a passive experience.
I listened to the airplanes, so I never heard the telephone. Is this sentence coherent?-- Yes, and so is this one: I listened to the tape, but I didn't hear the answer to the question.
First, I don't like the word 'entail' — all words are just what they are, with their own uses — so I am not going to follow up on that idea.
I looked at him, but I didn't see him.-- Yes, strange out of context, though it can be done, where look at is a simple glance but see is a greater recognition or understanding.