In Singapore there was a use of full field teletext to send back pages of information over the air as the return path if a Prestel like system. You had your own page address for “your” page
A sort if very early internet/ google like system
How actually did this work and what did the user end equipment look like?
I'm imagining you tuned the channel that had the full-frame teletext, entered text mode, dialled an access number and used the tone keypad on your phone as the reverse channel - either being given a page number when you called or having your 'own'?
Prestel in the UK ran using 1200/75 baud modems - but AIUI the system could have run with a 1200 baud half-duplex modem only, and a touch phone for the user control for the basic page selection (which were based on *800# style page numbers. However email was already becoming a thing - and a 75 baud modem back-channel allowed for alpha numeric text entry.