Here's a note that appeared in my weekly Scambusters email:

Alert of the Week: We're seeing a huge increase in the incidence of an old favorite scammers' trick -- caller ID spoofing.
As we explained in an earlier issue, spoofing is used to mislead victims into thinking an incoming phone call is from a legitimate source. Learn more here:

http://www.scambusters.org/callerid.html

Simply don't assume that a caller is who your ID service says it is. If you don't recognize the voice, don't give confidential information or agree to pay anything without independently checking.