Is one obligated to answer all the tzadakah mailings or phone calls? One does not know if its for real. People don’t know who is mailing the letter or who is on the other end of the telephone. Is it a middah of being miserly or just common sense.
If one is in doubt as to the worthiness of a charity collector, one should give a small amount of money (a perutah is sufficient, which really isn’t much!).
There is an obligation on the individual and on the community to help the poor and the needy, but if we were to give sizeable contributions to every request, we would quickly be left with nothing for ourselves!
Note that many mailings and phone calls are not from the poor themselves, but from organizations that deal with different social issues. Although contributions to such organizations are generally very worthy, they do not carry the same obligatory nature as a poor person who stands at the door asking for help.
Of course, not all causes are equal, and we have elsewhere discussed (see article on the site) the question of priorities in charity allocation. See also: http://www.dinonline.org/2010/08/12/laws-of-tzedakah-part-ii-who-to-give-first/.
Sources: See Bava Basra 9a. The Gemara states that we should appreciate the “fakers,” because they save us from sin: When we don’t give out of concern that the person is not truly poor, we do not sin. Yet, assuming that most requests are sincere, it is correct to answer them to some degree.