The Department of Human Services running Centrelink says these scenarios show reasons its clients might abandon their attempts to reach its call centres, denying there is a direct link between its notorious wait times and people giving up on the phone.

Five "case study" examples the department says show why people could be hanging up have drawn ridicule on social media and from critics who say the explanation is far simpler.

Among scenarios the department listed when responding to a Senate estimates question about abandoned calls were:

Centrelink's phone services have been a source of massive frustration for clients left waiting for prolonged periods to reach staff or hitting busy signals, which met 55 million calls in 2016-17.

Clients report abandoning calls after long waits, and Human Services last year named hold wait times, engaged signals and call disconnections among the top complaints it received.

The main public sector union said the department's response to the Senate question on abandoned calls was a bizarre attempt from its management to deny reality and described it as "frankly embarrassing".

"They may as well have told estimates that a dog ate their homework," Community and Public Sector Union deputy secretary Melissa Donnelly said.

"It's taken the department nearly two months to come up with the flimsiest of excuses for call wait times and abandonment rates that are clearly unacceptable.

"Try telling someone who's tried calling on their lunch break for help with the child care subsidy for five days in a row but had to hang up and get back to work that there's no link between waiting times and calls being abandoned. It's frankly ridiculous."

The union's members working in Centrelink, Medicare and Child Support knew the reality for callers who had regularly spent hours on hold and called dozens of times, she said.

Australian Council of Social Service chief executive Cassandra Goldie said mobile phone service could be patchy in Australia, but it wasn't leading to millions of abandoned calls to Centrelink.

"It is obvious that many people would be hanging up on a call because they are waiting on hold for too long. We all do that," she said.

"If you are trying to get through to Centrelink during your lunch break, or update your details between classes, you cannot afford to wait for half an hour to get through."

Opposition human services spokeswoman Linda Burney said Labor regularly heard from people who had been waiting hours to speak to Centrelink. She blamed staff cuts totalling 1200 last year and expected to reach 1300 before mid-2019.

"Their excuses are getting more and more fanciful," she said.

"At best the government lacks imagination, and at worst the government is in denial, if it doesn't think that people are abandoning calls to Centrelink because they've simply been waiting too long."

Department officials in May said average waiting time on calls had increased in the year to March 2018, from 15 minutes and 44 seconds up to 15 minutes and 56 seconds.

Human Services spokesman Hank Jongen stood by the case studies.

"These examples are based on feedback provided by staff from discussions with customers," he said.

"There is no direct correlation between abandoned calls and wait times as wait times may be only one of countless personal and environmental reasons why a call is ended."