I am pretty sure that everyone here remembers those old GTE/Verizon Airfone handsets onboard airlines (and perhaps its competitor AT&T Wireless Claircom on some aircraft). Verizon discontinued Airfone service on commercial airlines in 2006, and most airlines have now removed the handsets from their planes.
However, I remember noticing something different on some of the Airfone handsets on older planes: the handsets were released by inserting a credit card into a vertical slot. I remember seeing that version on DL's 727s and L-1011s; the 757s and 767s all had the newer version with the release buttons.
I was wondering, on the version that had the credit card slot latches, was that slot also used for payment? I know the version with the push button latches had a credit card swipe reader on the side of the phone for payment, however, I am not sure about the version with credit card slot latches. If the slot was simply for releasing the handset, I presume you were able to insert something other than a credit card to release the handset.