The BBC version was a small square of "static" broadcast in the corner of the closing credits of the Micro Show and you put and square plastic box (you could get from the show, I can't remember if you had to pay for them or not) with a light sensor over that "static" square and run the special software which take the data and save it as a file.
It wasn't terribly reliable but it was an interesting experiment by the BBC.
The "(BBC) Micro User" used to be published monthly, and had hardware project in each edition. Towards the end there were instructions on how to build your own acoustic-coupler modem where you pushed a regular telephone into two muffled cups. I think the only or main thing you could connect to was Prestel.
Prestel was my first online experience, over a 1200/75 baud modem (I just missed the acoustic coupler era, this was mid-80s) and led me onto dialing up many BBSs... much to my fathers gnashing of teeth when the quarterly British Telecom phone bill came through! Phone calls used to be expensive in the UK (even local).
I remember Micro User magazine, wish I still had my collection!
Does the BBC still have people in place doing experiments and R&D? I recall at one point they were researching/experimenting with audio codecs. I imagine this has taken a hit with funding cuts and general short-sightedness back home.
It wasn't terribly reliable but it was an interesting experiment by the BBC.