Phone Based Charity scams target good people and are perpetrated every day
In other instances callers may represent a "real" charity on paper but the concern you need to have is what is actually done with the money they slyly solicit from you.
The caller my indeed be from a legit charity but the problem lies in the fact that many keep a large portion (in some cases most) of each dollar they collect. (read Charity Navigator report about telemarketing)
A couple things you should always do when being solicited over the phone:
1. Never give out your personal information. This includes credit card account info and social security numbers.
2. Get the callers name and contact inform and inform them you will follow up on your own.
3. Research the organization on-line and share the data with a trusted legal or tax professional. If all meets your standards then you can contribute directly to the organization.
Point is there is no need for a telemarketer middleman. Even if they check out it is just a better practice to follow the three steps above a the very least.
Telemarketing is a red flag! Be proactive and know where and how your contributions are be handled