GC Services Refusing any correspondece I send Knoxville, Tennessee

GC Services sent me a letter stating I owed money for an old student loan.  I sent them a letter within the 30 days specified on their initial letter and they refused it.  They have refused every letter I send to them.  When I called to get their mailing addres they told me they would not give it to me unless I gave them my social security number, Name, address and phone numbers.  I believe this is against the Fair Credit Act.  I sent them a letter to the address on their letter under "mail correspondence to".  I have now gone and sent certified letters to every address GC Services has, filed a complaint with the Department if Education and The Fair Credit Act, with a copy of the denied correspondence I sent.  This company should be closed down