There are so comments already on this subject but I wanted to add one more. This is related to I think what Mazal said regarding phone solicitations and fake organizations. It is very important to check on the reliability of any organization whether by phone by requesting a letter sent by mail or a meshulach at the door even if they say they represent a Yeshiva or other mossad that’s well known.
For Israeli organizations you can check online with Olam Hatorah http://www.torahindex.com
It’s hard to stress enough the importance of this verification.