The Americans with Disabilities Act sets standards for recruiting and screening volunteers as well as employees. A points to a rich resource on working with volunteers, though the specific article mentioned seems to have been removed. Volunteers for charitable organizations can deduct unreimbursed expenses.
Great caution is advised.
Much depends on the context and the organizational setting. The key disadvantage to using telemarketing firms is the percentage of each pledged donation which they collect as a fee. There are probably more. Canvassers' are paid to do telemarketing. Reports must be filed on the money people are paid for work and taxes may be due, however small the payments and no matter how they are calculated.