Question

why is 'a'used before Ms.Bindu in the sentence A Ms.Bindu is on the phone though Ms Bindu is a proper noun?

Answer 1

Here 'a' is used before Ms. Bindu so as to show that the person is not known to the speaker.

Answer 2

    " A Ms Bindu is on the phone".
It means that    a person,  who calls herself  Ms Bindu, is on the phone.

It means that the speaker of this sentence is not familiar with Ms Bindu.  She identified herself on the phone.  The speaker is informing a third person regarding who called on the phone.    It is possible that the listener of this sentence, knows Ms Bindu.