The Ibo saying goes Never answer yes to a voice calling you if you are not sure who it is. It may be an evil spirit calling you. My mother used to tell me this all the time because for some odd reason I always thought I heard people calling my name (my name happens to sound like many things). I would get annoyed and shout Yes. Yes! Yes!! until my mother would say Yes what? To which I would reply Werent you calling me just a second ago? My mother would reply no, and add how many times do I have to tell you not to answer yes if you do not know or you are not sure who is calling you?
Another take on this superstition can be found in Chinua Achebes Things Fall Apart where one of the characters mother calls her from another hut and instead of answering yes she says is that me? (Achebe 41), which is a way to verify that the person calling you is human and not an evil spirit.
To me, not only is it a widely held superstition in the Ibo tribe but also it is a way to teach children how to answer with a yes and not a what and to properly identify who is calling them because it requires the person being claed to seek out who is calling them by not yelling across the room, yard, etc. to see who is calling them. It is a part of Nigerian culture as a whole to approach the person calling you, especially if they are an elder, by answering yes sir, mamn, mother, father, uncle, auntie, etc.
Achebe, Chinua. Things Fall Apart. Anchor Books: New York, 1994.