When the authorities could release the information about how many calls Rhea had made and to whom, they could have easily verified whether Disha had called up Sushant before her death.
It seems no. It looks like, Disha had not called up Sushant, nor they were close. But probably Sushant knew that she is also aware of some information which he is aware of. So he felt, if she can be killed for knowing that, next he can also be.
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