It's worth it to Telstra, to hold onto the real estate. They have effectively been been gifted a nationwide network of plots of land and structures that turn out to be a good match for 5G/6G/WiFi base station locations.

They are probably kicking themselves for ripping out so many phones in the past, thus giving up the locations.

They have a requirement to provide community phones, the cost of calls is trivial when compared with the cost of emptying coin draws and combating people trying to get a free call.

It's part of what's called the Universal Service Obligation (USO) for which Telstra (the dominant carrier) is responsible for delivering:

https://www.infrastructure.gov.au/media-technology-communica...

The money comes from a levy on telecommunications carriers (Telstra included, but also most of it's competitors). There was a bit of conjecture in the media that the "free" payphones were really being paid for by the other telcos, once the money Telstra contributes is discounted.