Our office recently needed to subpoena cellular phone records but could not determiine which carrier to subpoena. The following website allows you to put in the area code (NPA) and first three digits of the number (NXX) and obtain the name of the carrier.

http://puck.nether.net/npa-nxx/

Be aware that some of the carriers have changed names or assigned their numbers to another carrier or subsidiary, so you may still have to do a little more investigation, but this gives you a good starting point.

John - it also seems to work on regular "landline" numbers too!

We usually use -

http://www.primeris.com/fonefind

It too only gives the carrier who "owns" the line, but it gives a starting point.

For those who ever need records from SBC, they now have a website for law enforcement:
http://lea.sbc.com
You need a password though. I have one that LE can use if anyone needs it, email me at mhightower@wilco.org
(Since this forum is accessible by the public, I won't post it.)