it was my telephone number that miss white happebed to know.

你可能记错了!英语中的强调句型为it's.that.（如被强调部分为人,可用who代替that）.此句型可对谓语以外的成分强调.如：
I met my schoolmate in the street the other day.
It's I who/that met my schoolmate in the street the other day.强调 I
It's my schoolmate that/who I met in the street the other day.强调 my schoolmate
It's in the street that I met my schoolmate the other day.强调 in the street,不用 where
It's the other day that I met my schoolmate in the street.强调 the other day 不用 when
理解请采纳!