Yeah - this is something I discovered at UCSC in 1988, and for a few years I carried a mini tape recorder around everywhere with the sound of several dollars in quarters dropping into a payphone (recorded on to tape from the receiver of my phone in the dorm, as quarters were dropped into a payphone on campus). I was amazed when this actually worked (I had never heard of anything like this being done before), and, because it was the sound of the quarter dropping (the physical noise of it moving through the phone) plus the "beep beep beep beep beep", even if an operator was on the line this method could be used without being detected. My first international test was calling the Hard Rock Cafe in Helsinki (the operator was on the whole time as the "coins" were dropping).

Around 1992 I read about being able to replace the crystal in one of those little phone dialer things you could get at Radio Shack with one of a different megahertz to change the pitch of the DTMF sound so that pressing one of the numbers would make the nickel sound (I ordered this little part from Mouser Electronics). You would program the device to make five in a row to simulate the quarter. It was a bit more convenient than the little tape recorder (but, the downside of this technique is that if an operator was listening, it sounded obviously fake). A bit later I recorded the quarter sound from the tape to one of those little key fob recorders for even more convenience.

When I moved to Florida in 1993 this technique didn't work on most of the payphones around - I guess it was the start of payphone deregulation, and these phones had the mouthpiece disabled until the money was put in and the call was put through, so the sounds could not be played through it. There were a couple AT&T payphones around, and the technique did still work on those. I think around 1997 was the last time I successfully used this technique.