Can you ignore a ringing cell phone?

I noticed again this morning how difficult it is just to let a phone ring. Even though I was brushing my teeth, a perfectly acceptable reason not to, I felt guilty that I did not answer it right away. Are we conditioning ourselves ala Pavlovs’ dog (not the salivation part) to respond to the sound?
And when I am speaking with someone and the phone rings why do I feel a greater guilt if I ignore the call than if I excuse myself and answer it even though I am actually being, in my opinion, ruder to the person I am with?