More than 55 million calls from welfare recipients met an engaged signal last year and clients waited an average 30 minutes to have their calls answered by several Department of Human Services lines.
The 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.
But DHS officials blamed smartphone apps making multiple calls for the dramatic rise.
New department secretary Renee Leon told senators at an estimates hearing on Wednesday night that 2.8 million of its 5.8 million callers last year received a busy signal at some point in 2016-17.
The top 500 most frequent callers accounted for 685,000 busy signals, officials said.
"I don't think we can say it's more people trying to get through, rather more people repeat dialling," Ms Leon said.
"All of this is not to say we are not concerned about the number of busy signals people receive," she said.
The department was receiving data to find out how it could manage the problem.
DHS officials said the repeat dials caused problems for the department by distorting its understanding of client demand and inflating its call figures.
In 2014-2015, the number of busy signals received was 22 million.
Labor senator Lisa Singh told DHS officials the increase amounted to an "incredible number of calls."
Unemployed welfare recipients waited 30 minutes, clients calling the disability and carers line waited 28 minutes and aged pensioners waited 18 minutes on average to get through to a DHS staff member last year.
The DHS also blamed smartphone apps for a plunge in Centrelink's customer service performance after admitting in June more than 42 million calls to the agency received an engaged signal in just 10 months.
Human Services said in June that many callers were using mobile phone apps that allow them to redial "every couple of seconds".
In an unprecedented move, the DHS this month announced 250 of UK-based services company Serco's Australian subsidiary staff would take calls about payments in a three-year pilot program costing the government $51.7 million.
The DHS' annual report released this week revealed Centrelink received nearly 170,000 complaints last year as the agency was hit with the "robo-debt" maelstrom.
Among complaints, most (28.2 per cent) were made by clients tired of waiting too long for claims, not receiving updates on their progress or needing to contact the agency multiple times to advance them.
A quarter related to people dissatisfied with a decision, outcome or payment, while 16.2 per cent complained about hold wait times, engaged signals and call disconnections.