Spam by reputed organisation

I came across an ad a few months ago. The ad claimed to work for poor, and it is true. The organization is a reputed organisation named UNICEF. I have subscribed for a monthly donation. So where is the scam? It started since last couple of months. I receive regular spam calls from different numbers claimed to be from UNICEF, and I believe that as the spam callers know my details. They are now consistently nagging me to increase the donation amount. This is not a ethical way to call from different numbers and try to 'catch' the victim. I am going to cancel the mandate of monthly donation too.