Idk I don't mind that the very poor that don't have phones still have an option to make phone calls. I know a lot of victims of abuse use them as a way to make phone calls that aren't monitored by their abusive partner. The homeless can use them, or maybe your phone died. I don't recall seeing ads on them but I'm not exactly looking out for payphones.

Plus I'd hardly can a phone call "ageing technology". I mean yes it's an old technology, but still very relevant especially for those that don't have other means of contact

I think they are still around because the government told them they can't remove them. They are considered a public resource. Remember, Telstra used to be a government entity before it was privatised.

The Telstra pay phone boxes near me now all have massive ridiculously bright screens displaying advertisements. For sure that is their primary purpose now.

If they offer wifi I’m guessing the phones are VOIP?

Australia. I believe it's local calls only, and yeah I'd assume it's using VOIP