The benefit is they immediately hang up, then call you back on your mobile. The first interview question is "how did we do that?"

So if you get recruited, their surveillance partners like the five eyes don't know you are a spy. They are obviously sharing your call history with everyone, including people they might want to spy on some day.

It might just be to create a sense of allure. Taken from the movies. Like you are a spy now, dog.

It might be to keep other people around you from accidentally finding out you are a spy.

I'm guessing because they want the identities of their recruits to be secret, or at least difficult for other intelligence agencies to determine. Calling from a cell phone makes a very clear link to a specific person, while a pay phone would require other methods of intelligence gathering to unmask the identities of potential new spies. It is probably easy for a nation state level adversary to hack, or otherwise obtain phone records (especially if the call is somehow routed through the US).

There's even odds it's a response to the Gemalto hack; all three major AUS MNOs use Gemalto SIM cards, and at the time only NSA and GCHQ were specifically called out as being involved, not ASD. Canberra might be feeling a bit left out there. Alternatively, they might be afraid that PLA's Third Department have also compromised Gemalto or other upstream hardware providers, since Australia and China are in each other's region of interest.

Data retention and XKeyScores. Anyone with access can then see who works or wants to work for ASIS. Data retenton laws and information sharing can cut both ways.

They don't want other countries' spy agencies to know who they're recruiting.

What if there's some weird loophole among the Five Eyes participants that ignores payphones in favor of other landlines and cells?