No one else realised that Sailesh was talking to the Mhatre who killed his wife and because of whom Sonakshi is facing so much trouble.
There must be several persons with the same surname. Maybe that is why no one thought that it was the same person.
Rohit and Sonakshi knew that Sailesh worked for Mhatre and so they were able to deduce that it was the same Mhatre who called Sailesh.