Are you a member of True Credit? If you are, look at the bottom of the credit reports page, it list all creditors/collectors addresses, and phone numbers for some.
If you are not a member, you may want to consider joining to get all three reports every 24 hours. You can also try True Credit for 30 days free to veiw your Transunion credit report only.
If you are not interested in trying True Credit, then purchase a copy of your Transunion Credit Report about $10.00, or free if you haven't gotten your free report in less than a year. They have a list of all addresses-some with phone numbers.