It's the American Red Cross, one of their centralized call centers, either looking for blood donors or looking for donors to set up an appointment to donate.  They tend not to leave a message, and when you call the number back, they do not answer.  However, I called 2x more and did get thru to talk with a CSR from the American Red Cross.  He apologized and promised not to call for blood donations.  You can let them know for how long you would like the calls to stop for.