the only time you should answer your cell phone is when not answering it could conceivably result in someone bleeding to death. Even then you should apologize profusely to the person with whom you are talking and explain that you would never normally do this, but you would hate to have that death on your conscience.
Anything less than this is disrespectful to the person with whom you are sitting.
Listen, the whole idea of networking is to create mutually beneficial relationships. These are ones in which both parties are equally valuable to the whole. If you answer the phone in mid-conversation it's just telling them that you are always waiting for someone better to come along and you will abandon them at the drop of a hat.
Or the buzz of your cell phone.