Seriously?! Why answer a call just to tell someone you’ll call them back? If the incoming call is more important than the one you are on, simply tell the person you are talking to that you have to go. Don’t give some ridiculous “oh I have to take this call because…” excuse – simply say “I have to take this call” and be done with it. There is no need to justify why you have to take another call.
On the flip side, if you do not take the call, don’t mention it to the person you are talking to. Instead, send the caller directly to your voicemail if you can and focus on the call at hand. Some people are offended when you send them directly to voicemail. They shouldn’t be! They do not realize that you are doing them a favor by not wasting their time (or yours). Eventually they will realize that when they call you, you’ll answer if you can or send them to voicemail if you can’t. (Calling them back is up to you 🙂 )