Modular Technology’s Minimodem acoustic coupler was a method of turning a basic telephone into a modem without the need to build and have approved a plug-in telephone device. In the UK, all deviced connecting to a British Telecom phone line required approval which cost money. Here you simply placed the handset of the telephone into the acoustic coupler, which its self had a speaker and microphone.

The Minimodem was capable of 300 bits per second. Several models were built. Model 3001 and 3005 can be seen on the internet, but I am not sure which version this one is. This modem will turn audio data into serial data to be fed into a computer and vice versa.