These agencies consolidate your debt and make it an easy monthly instalment for you. They also try to negotiate with credit cards. They do so for a fee.
Other option is to not pay the debt. During this time , expect credit cards to keep sending you bills and reminders and ways to contact you.
Once it is not paid for a significant amount of time ( 18 months ) , the lender will "sell" your debt to a collection agency. You will start getting bills from collection agencies.
Collection agencies can settle for up to 40 % of the actual debt. So if you had 5 credit cards , you would have 5 different collection agencies trying to get in touch with you. You can call them and tell them that you cannot pay the full amount. They will offer you settlements which you can accept or decline. The longer the unpaid debt , the more the discount they will offer.
One very important thing to remember is that the unpaid amount will be sent to you on a 1099-c form . This means you have to recognize this as income. It is applicable to the year when the debt is settled.
In a nut shell , you owe 120,000. You don't pay. Credit cards keeps calling you. You don't pay. After 12-18 months , they handover your debt to collection agencies. Collection agencies will try to get in touch with you. Send you lawsuit letters.
You call and settle for say 50,000. You pay off 50,000 in 2016. Your debt is settled. But wait you will get 1099-C forms from different agencies totaling 70,000 ( unpaid debt ). You will have to declare that as income and you will owe tax on that. Assuming say 30 % tax you will have to pay up 21,000 as tax to IRS assuming no other income for simplicity.
SO what you did was pay up 50 + 21 = 71,000 and settled the debt of 120,000.
Your credit score will be much better than if you never paid at all.