The most common and most likely answer to this crossword clue is the 4 plus 8 letter word SIGN LANGUAGE
I've seen this crossword clue in The Sunday Telegraph
S
I
G
N
L
A
N
G
U
A
G
E
More clues leading to the result SIGN LANGUAGE
Buddy explains!
The straight part
We know that "Whereby text is digitally transmitted?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case whereby text is digitally transmitted? = sign language.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.