There’s a big difference between giving information to a stranger before the couple has even met, and withholding vital information once the couple is far along (or engaged!).
As far as I understand the halachos of shmiras halashon in regard to shidduchim, Aries is correct in not giving sensitive information to a stranger on the phone – she has loyalty to the person that she knows, and you never know what loose-lipped person will let that information out to people who shouldn’t have it. I don’t think you’re allowed to lie, but you are allowed to avoid the questions. If you want to make sure that people are being completely candid with you, call people that *you* know, or have someone that knows both you and the reference make the call.
However, in Ofcourse’s case, the fact that people knew that a major issues hadn’t been disclosed and nobody said anything about it is a BIG problem. It *was* the chosson’s/parents’ responsibility to divulge the information, but if they didn’t, someone else should have alerted the kallah to the issues. Ofcourse, did you ask a shailah at that point?