By Bob Howard
BBC Radio 4's Money Box
Ofcom is consulting with the industry to update the guidelines
A privacy campaign group says tougher guidelines are needed for banks using technology known as interactive voice messaging.
This uses automated messages to call customers who have missed a payment and advises them on their options.
Privacy International says without adequate rules, the system can be used to harass customers into paying debts.
The banks deny this and say they follow guidelines laid down by the telecoms regulator, Ofcom.
Lloyds TSB, NatWest/Royal Bank of Scotland and Halifax Bank of Scotland all use this system to varying degrees.
Allan Cook from Cardiff started receiving calls from NatWest earlier this month, trying to contact his son.
Eventually he phoned the bank to ask them to stop calling, as his son was not at home, but he says the bank refused.
He told BBC Radio 4's Money Box he was angry:
"This just went on day, after day, after day, six or seven times a day at least.
"It went on for about eight days."
Simon Davies, director of the campaign group Privacy International, says it is vital for clear guidelines to be issued for the industry to follow:
"I wouldn't object to a single call in a 24 hour period.
"If it goes beyond that, I would certainly argue there's an element of harassment involved."
NatWest said it would not expect to call customers as much as seven or eight times a day, but it might two or three times, although no one would be called more than five days in a row.
It said only a small number of customers were contacted this way and, throughout the call, customers can choose to speak to a member of staff.
Lloyds TSB which uses a similar system said it would call up to four times a day, but no more than four days consecutively.
Both banks said they follow guidelines laid down by the telecoms regulator Ofcom.
What is unreasonable?
Ofcom is currently in consultation with the industry in order to update these guidelines.
It said it thought eight messages a day might breach Office of Fair Trading (OFT) guidelines on harassing customers over debts.
The OFT said the Consumer Credit Act lays out that banks cannot contact debtors at unreasonable times and at unreasonable intervals, but it said only a court could decide what "unreasonable" meant in practice.
Companies which have been selling this technology to UK banks feel it is being used responsibly.
David Clapham is from Talkingtech:
"We refer to our service as a friendly reminder, we're not acting as a heavy solution on the phone."
Bob Walsh used to work for two major high street banks and is now a consultant advising them on debt collection.
He says banks using these systems must do so with sensitivity.
"Most of the customers in these situations are continuing customers, and once you've annoyed a customer once, you're probably going to lose him."
Ofcom expects to publish its new guidelines on how these systems are used in June.
BBC Radio 4's Money Box was broadcast on Saturday,
31 May 2008 at 1204 BST.