Speaking of that: I did it a few times back in '99. Had a notebook and one of those "ConnexCoupler" acoustic modem adapters. Same thing used by the actors in the movie "Hackers" to call the mainframe from payphones in Grand Central. Anyhow, I had this (now-defunct) weird local freebie dialup ISP that required you to call in using their software, enter the called-from number and then have their network call you back before granting internet access. I used that to my advantage to surf from payphones. Went to a rather remote payphone, dropped in 35¢ and called its number from itself to verify it takes incoming calls (as it was the only phone there). Got busy signal, so far so good. Velcroed the receiver to the coupler, dropped in 35¢ again, dialed ISP, punched in payphone number, let it disconnect then held the hook down. About 10 seconds later, it rang! Released hook & clicked "answer call" button on software (the ISP's software popped up this button to answer the call -- think it just sent an "ATA" to the modem), popped in userid/password was online! Only limitation was a 60-minute session before you were disconnected and had to go thru the whole process again! Of course, today, all Verizon payphones are 50¢ local unlimited, so I can my local ISP and be limited only by the charge in my notebook's battery &/or the 9-volt in the coupler.