Waangi is the response most typically heard in Uganda to a person calling someone else.  It can't be translated, although it sounds similar to "what" or "uh-huh." But it's not the same.  
Oh, and Wapi is "where" in Swahili, but it makes it's way in to the vernacular of a host of East African languages as a manner of interjection expressing a mixture of disbelief and amazement.  
One last item you'll bump up with in your rambles here is "waat."  Waat is the worlds most useful term.  It means, well, "what," but it really means more than that.  If you're a Ugandan and you can't call something to mind, you just say "waat, waat" until it comes to you.  Similarly, if you are talking about things in a series, such as a list of items you need for lunch, you might say, "We need matooke, beans, waat, waat."  So it functions like et cetera does for English speakers.