Financial firms have been urged to cut high-rate customer lines after a study found almost three quarters are costly 084 or 087 numbers.
Which? found that 177 out of 242 customer or complaints lines for services such as current accounts, loans and credit cards - or 73% - were 084 or 087 numbers.
The companies included leading high street banks and building societies such as HSBC, Lloyds Bank, Nationwide and TSB Bank, credit card providers American Express, Capital One and Tesco Bank and insurers Aviva, Churchill and Direct Line.
The watchdog found that four in 10 people (39%) prefer to call financial firms with an inquiry and nearly a third (31%) would rather complain by phone.
However nearly all of the credit card providers studied (95%) use 084 or 087 numbers for complaints or customer service help lines and 89% of current account providers use them for complaints or customer service help lines.
Existing customers are also being charged more than new ones, with free 0800 numbers used for 52% of sales or new customer lines compared with just 26% for existing customers and 21% for complaints.
Barclays and Barclaycard have announced that they will offer a freephone or basic rate number for all customer help lines, and Which? called on other providers to follow their example.
The EU Consumer Rights Directive ban on the use of expensive numbers for customer help lines comes into force next year, but financial firms are excluded.
Which? is calling on the Financial Conduct Authority (FCA) to clarify existing rules to stop financial services companies from using high rate numbers on complaints lines, and change the rules so they also cover customer help lines.
Which? executive director Richard Lloyd said: "Millions of us prefer to deal with our bank on the phone, yet we are expected to cough up for a costly call when we do.
"We applaud Barclays and Barclaycard for breaking from the pack on high rate numbers and want to see other financial firms follow their lead.
"It's not right that financial companies are being let off the hook. The FCA must act now to put an end to costly calls in this sector."
Ashok Vaswani, chief executive of Barclays Retail and Business Banking, said: "For many customers the telephone is the most convenient way in which to contact us, so it's right that we have taken this step to ensure that no customer need dial a premium or high rate number simply to speak to us."
:: Populus surveyed 2,070 adults online between August 30 and September 1.
Richard Lloyd, Which? executive director, said: "It's great news that NatWest and RBS are doing the right thing for their customers by dropping costly calls. The new leaders at RBS have promised to renew the banks' efforts to improve customer service, and this is a very welcome start.
"With two of the biggest banking groups now leading the way by offering freephone or geographic numbers, we hope this is a tipping point for the banking sector - there's really no excuse for other providers not to follow suit."
A British Bankers' Association spokesman said: "All banks are actively looking at how they can reduce costs for customers. We expect to see many banks changing to use local numbers for complaints in the near future and it is good to see that some banks have already committed to doing so."