I also received a call from this #. It is a VALID company. They call about student loans in default.You can go to thios government website to verify-
http://www.ed.gov/offices/OSFAP/DCS/index.html

Here is the company info-
Diversified Collection Services, Inc.
P.O. Box 9049
Pleasanton, CA 94566-9049
(888) 335-6267