Its probably a spoofed number to begin with. Be careful calling back numbers though, some of them lead to getting a very ugly bill or porn ..

https://www.ftccomplaintassistant.gov/#&panel1-1
https://esupport.fcc.gov/ccmsforms/form1088.action
https://complaints.donotcall.gov/complaint/complaintcheck.aspx?panel=2  <telemarketing only>
http://www.consumerfinance.gov/complaint/