String and Dime ( yes, that's dime, kids

) didn't work. Neither did unscrewing the mouthpiece as they were either glued or sonically welded. But I may have known about some individuals having some success with a deposit of a nickel and a red-box. You see it could be determined if money was physically deposited into the phone, and then resulting tones played from the red box would add additional "credits" ,as it were, to the call. IIRC that kind of action ended mid to late '80's in Florida. Radio shack actually sold a DTMF keypad that with a little hacking, any EE student could easily turn into a red box. BTW did I mention I was an EE student in 82-84 in SW Florida? Have any of you actually seen a payphone in the wild these days? I don't recall seeing one for a very long time.
Just sayin'
Scam