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I think they wanted to show the connection ruditha share. When they both together when one is upset they used this coconut shell to talk to each other. Now it had become a sweet memory of their love.
Bonditha is too young to understand that coconut shell is not another phone but Ani, being barrister using it means ,for him it had become a symbol of their love. Though they are apart they are connected by heart and mind. No one can take away that connection .
As I shared in wristwatch scene symbolism Ani cannot understand the clues infront of him, Shre. I wonder who will point them to him.
Sampoorna ,Batuk ,Kaka , Saurab ,Bihari or Koeli . Maybe Bidu even . They will do unintentionally . He is too depressed now to understand his own and after his barrister brain wakes on the last day only.