You do not give a lot of specifics, but hopefully you know for certain what this dispute is about.  The notice sent to you by the debt collectors should contain certain specific information.  The following information (below) is taken directly from 209 Code of Massachusetts Regulations 18.18, and describes what the debt collector should have sent, and your rights to have the debt validated (receive proof that the debt is legitimate):
 
"1. Within five days after the initial communication with a consumer in connection with the collection of any debt, a debt collector shall, unless the following information is contained in the initial communication or the consumer has paid the debt, send the consumer a written notice containing:
 
a.  the amount of the debt;
 
b.  the name of the creditor to whom the debt is owed;
 
c.  a statement that unless the consumer, within thirty days after receipt of the notice, disputes that validity of the debt, or any portion thereof, the debt will be assumed to be valid by the debt collector;
 
d.  a statement that if the consumer notifies the debt collector in writing within the thirty-day period that the debt, or any portion thereof, is disputed, the debt collector will obtain verification of the debt or a copy of a judgment against the consumer and a copy of such verification or judgment will be mailed to the consumer by the debt collector; and
 
e.  a statement that, upon the consumer's written request within the thirty-day period, the debt collector will provide the consumer with the name and address of the original creditor, if different from the current creditor.
 
2. If the consumer notifies the debt collector in writing within the thirty-day period described in 209 CMR 18.18(1) that the debt, or any portion thereof, is disputed, or that the consumer requests the name and address of the original creditor, the debt collector shall cease collection of the debt, or any disputed portion thereof, until the debt collector obtains verification of the debt or a copy of a judgment, or the name and address of the original creditor, and a copy of such verification or judgment, or name and address of the original creditor, is mailed to the consumer by the debt collector."
 
If you follow the steps outlined above you should be able to stop the collection efforts while the debt is verified.  You can find these and other relevant regulations on the web site of the Massachusetts Office of consumer Affairs and Business Regulation.  If you continue to have problems with the debt collector, you can try their Consumer hotline at 617-973-8787.  Also, the Attorney General of Massachusetts has published a helpful guide to your rights as a debtor: http://www.mass.gov/Cago/docs/Consumer/fair_debt_collection_100407.pdf.
Good luck, and please tell your friends about The Forum.