Yesterday the Herald reported that from next month Telstra landline customers would be billed in 30-second increments for STD and international calls, potentially delivering millions of dollars in extra revenue.

Telstra has defended the move, arguing the changes would make for greater efficiency and bring landline billing into line with fixed-to-mobile billing, which was changed from per-second to 30-second blocks in November.

At the same time, an unknown number of Telstra mobile phone customers have stopped receiving monthly bills, and have not seen a bill from Telstra this year.

Max Cray, who has four mobile phone accounts with Telstra for the biofuel company he runs, believes he owes Telstra thousands of dollars but has been unable to pay a cent since November.

When Mr Cray contacted Telstra he was told "thousands" of customers with mobile cap accounts were in the same predicament and that the problem would be fixed by January.

In December he was notified there had been a delay in processing bills and that once the problem had been sorted out customers would be offered payment plans to help them cope with hefty accumulated debts.

Mr Cray is still waiting for a bill. "Telstra's attempts to amalgamate its billing systems has proven to be a monumental failure," he said.

Telstra yesterday declined to say how many mobile phone customers had been affected. It defended the new billing system as being "world class".

A Telstra spokesman, David Luff, would concede only that "some" mobile capped plan customers had been affected and that the problem was being "actively managed".

In a statement, Mr Luff said: "Sometimes during our normal business processes, we become aware of issues that may impact a customer's bill. We can then choose to delay the bill for those customers to ensure the bill is accurate before dispatch.

"Where a bill is delayed we communicate with customers, and do our utmost to issue the bill as soon as possible. We also provide appropriate payment terms to affected customers."

Critics of the landline billing system made their complaints heard yesterday, on talkback radio, in chat rooms and in letters to Fairfax newspapers.

National Seniors Australia described the move to charge landline customers in 30-second blocks for overseas and STD calls "cynical". It would hit senior citizens hardest as they knew less about alternatives to landline calls such as phone cards or internet systems such as VoIP (Voice over Internet Protocol).

The organisation's chief executive, Michael O'Neill, said Bureau of Statistics figures showed that while nearly half of Australians aged 65 to 74 had access to the internet, just 28 per cent used it.

The chief executive of the Consumers' Telecommunications Network, Teresa Corbin, described the billing change as "blatant gouging". Telstra had failed to offer a plausible justification for the change.

Telstra's main landline competitor, Optus, said it had no plans to switch from per-second to 30-second block billing.