StepChange said high-cost credit, such as payday loans and fee-charging debt management services, should not be sold via unsolicited calls.
A survey found a third of its clients received more than five calls a week.
It estimates that buying such services added an average of £1,052 to its clients' existing debts.
"The problem is particularly serious among the most financially vulnerable in our society," said Mike O'Connor, chief executive of StepChange.
"When someone is already in financial difficulty, they may be at their lowest ebb and feel they have no option but to make quick decisions through desperation, which can bring devastating consequences.
"Before taking out any financial product, people need to know whether it is right for them, work out if they can afford it and shop around for the best deal, but unsolicited phone calls can take these key decisions away from them.
"It is not a good way to sell credit or financial services and it is certainly not a good way to buy them."
The call comes despite research from consumer group Which? suggesting that borrowing money on an unarranged overdraft from a bank can be more expensive than taking out a payday loan.
Borrowing £100 for 28 days from a payday lender would bring a maximum charge of £22.40 compared with a charge of up to £90 for an unarranged overdraft, Which? said. The banks argue that such overdrafts should be a last resort and there are cheaper alternatives.
Russell Hamblin-Boone, chief executive of the Consumer Finance Association, which represents some payday lenders, said: "This small survey by StepChange is misinformed and draws the wrong conclusions.
"Cold calling is carried out by unscrupulous firms who purchase people's data, not lenders. The sector is regulated and CFA members do not cold call, do not lend to people who cannot afford credit and ensure all lending is responsible. Regulations exist to protect borrowers and there is no need to further restrict how lenders market to their customers."
Credit should be "bought, not sold", a debt charity has said, adding that cold calls offering high-risk financial products should be banned.