A strange number just called and asked me if I had any plans to buy a house in the near future.

It shouldn't be a liar. It's probably a sales call from a developer looking for a customer to buy a house. Just say you are not interested, even if you are really a liar, you can't cheat anything.

If he knows how to address you, your personal information may have been leaked. As for the leaking channels, there is generally no way to trace them. The property you have used, express delivery, and app are all possible.