It's equivalent to "I heard that...." and you're asking if it is true/correct. Adj / (Verb + ㄴ/는) + 다면서요 e.g. 아팠다면서요? = I heard you were hurt (is it true?) 전화 한다면서요? = I thought you said you will call (is it true?) 하교에 갔었다면서요? = I heard you went to school (is it true?) You can drop the 요 for informal speech.

I've heard that~~ people says that ~~ it's utilised when we borrow other's words.

If you say ~다면서요?, you mean you ask someone again something you have heard.