By payphone i mean browser based softphone with a one connection limit . You will need mic and speakers. Goal is for it to be in HTML 5 but mic isn't fully exposed yet in the standard. Might have to go java based. I really want it so that nothing needs to be downloaded to use it. I'm going to start with one "booth" and hopefully scale up from there.

Basically when you hit the insert coin button you will get a new bitcoin address to send BTC to if left blank. If not blank and an address is entered, a check will be made to see if that address contains funds and is actually mine. If it passes these checks a phone call can be made.  If you want to verify encryption you will have to sniff the packets i guess or call a ZRTP enabled  device/softphone and verify the codes match.