Fortunately most taxi/airport phones aren't that hi-tech in the UK, so it's free call's a plenty. Tapping or tones work, lovely.
As for payphones, no one should
pay to use them (that's the fun of them!). A common crime in the UK a while back was to hi-jack money before it get's to the collection box at the bottom.
Not sure if your phones are the same, but what people used to do was to hack-saw a gap in the tube/passage leading to the box (top->bottom: Phone - small tube - money box) and put something there to block money actually making it into the box at the bottom, do this in a busy town centre with lots and lots of payphones and the small passage filled up quickly.
Of course it's no where near as rewarding as taking the whole money box,
but you can make your collection in broad daylight with a slight bit of cover from a jacket or similar, and collections can be as frequent as you like.
Still though, seems pointless to me, my fun with payphones has always been abusing them to get free calls/dial-up rather than actual currency