collection agencies are most definately involved when student loans go into default. Federally insured loans are never sold to CA's....they are assigned to CA's by the guarantor. The department of ed has at least a dozen CA vendors and most FFELP lenders have several CA's working for them. It is the guarantor that files the lien with the IRS. In some cases a letter is sent out warning of impending offset.
In order to stop IRS offset, you need to take care of the loan. Consolidate it, rehab it...get back into repayment. It is too late now for your 2006 return but for future years, get that loan into repayment!