In Japan, they would just accept DTMF tones from anything that would generate them. There was all these people using hacked cards that they installed countermeasures against which were hilarious because people using these hacked yakuza cards would keep one foot in the doorway in case the fabled alarm would go off, i knew a guy who did this thinking naaah it would never happen to me. NOPE happens to him, he had to squeeze himself out of that phone booth and run after a red light started flashing and the door slammed shut on his shoe... Literally just playing the DTMF tones into the handset would have gotten past that on every 'grey phone' out there (The ones that advertise ISDN connectivity). Wouldn't be surprised if that still works if they still have those phones anywhere.
The other way I did it in another country was a telephone company test line (toll free!) which would give you 30 seconds of silence then a dial tone (presumably 'remote'). From this dial tone you could call anywhere in the world. We got some list that phonelosers used to make and called places like the president of Kenya.
This guy mentions using a payphone which accepts incoming calls to get the internet as a kid in the 90s. Those which still rang on incoming were mostly gone by my time but a few were still configured to act like that. Was fun to sit down the road with a cellphone and watch people be scared to pick up the line after watching it ring for a few seconds.