The Human Services department has admitted more than half a million more client calls have met engaged signals compared to last year - but denies staff cuts are to blame.
More than 33 million calls to Centrelink received no answer between July and January, an increase of nearly 550,000 on the same period the previous year.
Department bosses told senators of the rise at an estimates hearing last week, saying that despite the result there had been a year-on-year drop of nine million unanswered calls in the last five months.
The giant agency last year recorded more than 55 million calls reaching engaged signals, a figure it blamed partly on the use of mobile phone apps that redialled their lines multiple times.
Last year's busy signal figures, covering July 2016 to June 2017, were a near-doubling on the previous 12 months when 29 million calls were met with the beep-beep-beep tone.
Human Services secretary Renee Leon said on Thursday that until October, the department had been dealing with "complexity" in its queue system and that the latest figures would not reflect improvements.
Labor senator Lisa Singh also raised new figures showing 339,000 Centrelink clients waited more than 30 minutes and 167,000 waited more than an hour to be answered in 2016-17.
"Can I say at the outset that we are not at all either complacent or satisfied with the figures that you're quoting," Ms Leon said.
Staff cuts were not behind the rise in unanswered calls because there was no reduction to its public servants attending to its phone lines in recent years, she said.
"The number of staff on the phones has been broadly comparable for the last three years," Ms Leon said.
"I think there's a reduction of a few hundred in the past year, and that's been only because we really took a big attack on processing backlogs, and that meant we shifted the balance of staff between phones and processing for a particular period at the beginning of this financial year.
"But there isn't any resource based reason that means we've reduced - we haven't reduced the number of staff answering the phones."
Problems with technology and auto-dialling calls had caused the problems, Ms Leon said.
"You can certainly just keep throwing more staff at it, but it's a much more efficient use of taxpayers' money to address the underlying problem so people can get through."
Senator Singh challenged Ms Leon's claim, saying that she had only compared the last three years, when the department had left tens of millions of calls unanswered.
Labor on Wednesday called for more Centrelink staff to improve call wait times while the Community and Public Sector Union has previously argued the number of unanswered calls has climbed rapidly as the Coalition government has cut DHS staff.
Greens senator Rachel Siewert said more than 33 million busy signals in the year to date was an "astronomical number".
"The government is trying to make out like things are improving but the figures don't show that at the moment," she said.
"For the department to play down the role of under-resourcing of staffing is just ridiculous.
"It is clear the department needs to be better resourced so it's not scrambling. Yes, there are problems with the technology, but it is a multifaceted problem and more broadly the department needs more resources to cope with this issue."
Under last year's federal budget, Human Services was slated to cut about 1200 jobs.
Since a three-year, $51.7 million trial using Serco to answer calls to Centrelink started on October 30, the contractor's staff have answered 981,000 calls.
The department said Serco took calls about online support for myGov, general online support, questions around BasicsCards and reporting of employment income.