If the bank has referred your case to its recovery agencies then bank has to inform you by letter mentioning the contact details of the recovery agency. Bank has to maintain the list of recovery agencies on its website. You may check this list.
If the recovery agents are coercing/threatening/ forcing/pressurizing/ intimidating you or are calling you frequently from inaccessible/non traceable numbers you may record the calls and lodge a police complaint, under acknowledgment.
RBI, BCSBI, IBA have issued guidelines on Code of Conduct for CC companies.
During your course of your transactions you must have noted and recorded lapses at the end of CC Company this HDFC.
Highlight this in writing and transfer the onus on this HDFC.
Agree to pay the principle amount only.
However obtain a confirmation in writing on letterhead under original seal and signature by redg. post from competent employee of HDFC bank stating that HDFC bank shall withdraw your name from list of bad customers maintained by bank, list of defaulters sent to CIBIL and shall clean your credit history and shall provide you a certified copy of clean CIBIL report showing name of member of CIBIL i.e. HDFC bank, MNNF list, Equifax, Experian and any other credit rating/bureau and this settlement shall have no adverse bearing on your credibility/loan taking ability in future.
It shall be appropriate to approach a lawyer specializing in consumer matters and CC cases.
Some court judgments in CC cases are enclosed.
There are many informative threads involving Credit Card harasment at LCI.
You may go thru these and prepare your defence.