Surely there’s a vendor that will sell you a v.22bis modem that works over VoIP if that’s what your two mainframes need to sync up, and you’re buying the multimillion dollar support contract…

>'Dial up' modems need a PSTN line to work

Cable company here (US) still sells service that has POTS over cable modem. Just plug your modem into the cable modem tele slot and you have a dialton. Now, are you getting super high speed connections, no, but that's not what you need for most hacking like this. Not that I recommend hacking from your own house.

I should have restricted that statement to include the United States of America. PSTN's are still utilized, deployed and actively sold in most of the US. As a side note I recently tried to get a telco to remove a phone line and two poles and they refused to do it. Their excuse was that they might one day run fiber over it despite there already being a fiber network here. I hope they do as my fiber ISP really does need a competitor. If they really do run the fiber over those poles vs burying it that would be amusing.

To your point I am sure some day the US will stop selling access to the PSTN but some old systems will hold on for dear life, government contracts and all. Governments are kindof slow to migrate to newer things.