The clear box works on 'post pay' coin phones, pay phones that require money only after the connection has been made. The way this works is: After the connection is made the mouthpiece of the phone is muted but not the earpiece, free calls to dial-it services can be made with these phones.
In order to talk to the person you called without paying (NOTE: It is against the law to do this!!) Take yourself down to your nearby electronics store and get a four transistor amplifier and a telephone suction cup inductive pick-up. Put the pick-up on the earpiece and plug it into the output of the amplifier, and plug a microphone into the input. You then talk into your microphone and listen normally through the earpiece.
Radio Shack sells an item that wont need much modification,and that should work the same as the above construction. It is their 'Portable snap-on handset amplifier' (Part # 43-238) Their description says to put it on the earpiece and it will boost the callers voice to five times the normal level. In order to make this function like a clear box one would have to take the amplifier apart and remove the internal speaker, in it's place, connect the suction cup inductive pickup. Place the pick-up (which is now the speaker) to the earpiece of the payphone, and talk into the microphone of the amplifer.
The line will not cut off, and will wait forever for you to put the coins in. Note also that these types of payphones are not connected to the TSPS in the same way as normal payphones. The phone does all the charging and not the Central Office. It is because of this, that a phone connected to the lines BEFORE the payphone would act just like a normal phone. So get out your smallest phone, cut off the jack and strip the wires. connect the wires to two alligator clips. Then you can clip onto the payphone's wires BEFORE they connect to the payphone, you then have a normal telephone line that you dont pay the bills on!