Read all about it:
Another of the earliest devices designed for the Apple II came from Apple Pugetsound Program Library Exchange (A.P.P.L.E.). They were involved in distributing Integer BASIC programs on cassette to members of the group. To make it easier to send those programs to the person responsible for duplicating the cassette, Darrell Aldrich designed a means of sending the programs over the telephone lines. There were no modems available at the time, so his “Apple Box” was attached to the phone line with alligator clips and then plugged into the cassette port on the Apple II. To send a program, you first called up the person who was to receive it and got the computers on each end connected to the Apple Box. The sender then used the SAVE command in BASIC to tell the computer to save a program to tape. In actuality, the program was being “saved” through the cassette “out” port to the Apple Box, and onto the phone line connected. At the other end of that phone line, the data went into the other Apple Box, which was connected to the cassette “in” port on the other Apple II. That computer was executing the LOAD command in BASIC to “load” the program from the Apple Box. A.P.P.L.E. sold about twenty of these Apple Boxes at $10 apiece.