Mohiyul Islam"telling window"

It's meaningless without its context. Please provide the whole sentence.

CJ

According to UNICEF, the health of women is a telling window on female inequality and it is most telling at the most vulnerable time of life: infancy and early childhood.

Mohiyul Islama telling window on

a revealing view of

CJ

A bank teller is a person who works with customers in a bank. They work in an area that is sometimes called 'a teller window'.
Is this what you mean?

Is it then an idiomatic phrase or not ?

Mohiyul IslamIs it then an idiomatic phrase or not ?

"a telling view" is idiomatic. It is how native English speakers write. It sounds natural.
It is not an idiom, however, because the words in it are to be taken literally. There is no unusual or unexpected meaning for those words.

CJ

May be so. I'm not sure.