This is a pun on the phrase 'for whom the bell tolls' which historrically means because none of us stands alone in the world, each human death affects all of us. Every funeral bell, therefore, tolls for thee". However, 'Belle' is used in place of 'bell' because it sounds similar and because it means a beautiful girl or woman, especially the most beautiful at a particular event or in a particular group, likethe woman in question here
Pablo and Jose waited for a signal from Pilar to join her in the Church Tower , but neither responded to the chiming because they didn ' t know for whom the belle was tolling .