The communication fails to function between Ali and Parvez because their individual views of Islam are diametrically opposed, and neither is willing to compromise personal conviction for the sake of peace.
When Parvez finds out that Ali has been giving away or throwing away his belongings, he is particularly worried. He tries to talk to his son, but finds that Ali strongly resists his well-meaning solicitude. Parvez's own experience with religion was fraught with condemnation and pain; his own taxi friends have also observed the hypocrisy of local mullahs who indulged unnatural proclivities toward the boys and girls in their charge while proclaiming righteous living to everyone else. It is clear that Parvez relishes living in England where he is free to live according to his best conscience. However, Ali having never experienced the pain of religious tyranny, wishes to live apart from what he considers the excesses and moral corruption of Western society.
What Parvez views as freedom Ali considers heresy. While Parvez considers himself a good man who deserves an occasional alcoholic drink, Ali is horrified by such decadent indulgence. Ali's 'aggressive confidence' is intimidating to his father, who thinks that his son has 'swallowed someone else's voice.' Indeed, Ali can't 'hear' his father's voice because other more eloquent 'voices' have supplanted Parvez's awkward and vague articulations about living a good life. Ali is simply not satisfied with Parvez's secular views; his thirst for meaning and purpose is at the crux of his decision to pursue a fairly radical interpretation of Islam. Also, Ali's uncharacteristic and blatant disrespect towards Pervez has blind-sided the older man. As it stands, communication does not function between Ali and Pervez because:
1)Neither party wants to compromise personal convictions.
2)Neither party is willing to respect open dialogue without recrimination.
3)At this point in time, neither party trusts the other.