Yes, unless they are a business person and waiting for an important call. But then that person would have told you ahead of time that he/she was expecting an important call. Answering a cell phone while talking to another is like interrupting your conversation and beginning a new one with someone else in the room. Totally rude!!
It may be very important. Relax. You're not the most important person on earth. I think it's selfish of you to expect everyone to treat you like a SPECIAL person.
There's only ONE person that can keep ME from answering my phone. Maria the massage girl. When SHE'S working, NO PHONE CALL GETS THROUGH.