With out the use of the Internet to take advantage of free Voice over IP I don't think you are going to find such a service.
My suggestion would be to look at your board members and friends of the organization, and see if any of them are from a large company that may have their own conference bridge that they would be willing to donate some conference call time and access to you.
Alternatively find the donations to pay for the conference calls I've seen like 5 cents a minute per participant.
If you have a the number of phone lines coming into your organization that you need for your conference calls you could buy a phone conference bridge and host the calls your self. You would have to get 800 inbound calling to not have any long distance visible, but some one still has to pay for the call.
There is no such thing as a free lunch
Dave