Hey Nicole,
We, too, use prepaid long distance cards for one of our long, long,long-term research projects. Originally we were going thru UMN's NTS, but they alerted us to the discontinuation of their contract with Verizon and counseled us as to alternatives. Our callers may not be in the building when they do their surveys which are necessary for the grant objectives. The PI has one card for one grant and another for a different grant. The caller typically focuses on one grant at a time - so they are confident there is no cross-over. They do log the subject called, but not the duration of the call. When the card runs out, they reload.
Are you now saying this is not acceptable. What are the alternatives then?