Dh was a victim last year. It started with a phone call from a cell phone company, checking on some conflicting info when someone went to open an account.
I suggest you contact Cingular Wireless directly. Hopefully you've taken down some info from the annoying collection agency- like the account number (phone number) of the fraudulent cell phone account. Give Cingular Wireless a call and ask to talk to their billing department. Once they figure out the account info (and write it down - keep a log of this stuff!), ask them to open up a fraud investigation on the account. Ask them to spell your name, confirm the address and the last four digits of your SS#. The fraud investigation can take anywhere from a couple of days to a week or more. They should send you an affadavit to sign and send back, and then you need to make sure that they follow up with a letter that tells you that you are not responsible for the debt. Keep a file!
Once you've figured out for sure that someone has opened a cell phone account in your name, you'll need to call one of the big three credit reporting agencies. Try
http://www.experian.com/ for starters. Somewhere on their site they should have an 800 number to call to report identity theft. You can put a fraud alert on your credit report- and doing so should trigger fraud alerts on your credit report at the other two agencies. When I had to call, there was a recording afterwards that told me that the info would be passed along to the other agencies, and sure enough within a few days I had letters from all three agencies telling me that a fraud alert had been put on dh's credit report.
You can also get a free copy of your credit report each year. You only need one - so pick an agency, go to their site and print it out. You can do that every four months, going to each of the agencies once every four months to encompass the full calendar year for free.
On dh's credit report, we saw two credit checks for cell phone companies. Someone opened three different cell phone accounts under his name within a few days. Unfortunately, we didn't find out until well after the fact about one of the other two - because of the lag time between the account opening and the account being unpaid and sent to collections.
In all three cases, we were able to get the stuff cleared up fairly quickly, except for a Qwest cell. It took many months and many phone calls to get that straightened out. They never checked dh's credit report, btw.
When you do confirm identity theft, call your local non-emergency phone number for the police and file a report. You'll get a case file number that you'll be able to give to the cell phone company.
On collection agencies- if you explain that you've been a victim of identity theft, they'll usually give you the contact info for the original debt (the cell company). If they're nasty, hang up, call back, and try again (I had to do that a few times - collection agency people can be quite nasty).