There are two linear very short character scripts in popular use today. One is the venerable Morse code, used and in fact mandated for independent radio operators/license holders until very recently, about a decade back. It dates back to the mid 19th century, and was developed in partnership with popularising the telegraph as a reliable remote communications system
The second is Tap2Talk, about half a century old, and which actually has no commercial usage or purpose. It was designed as an pro-freedom communication code for use by people who did not know Morse and, kept in isolation captivity, were denied any contact with fellow humans.
Both methods exploit vibration, either through the air, electromagnetic waves or through solids. The third option means that no electronic or electrical device is actually necessary to learn these scripts.
And it is this third option that means that the learning part of knowing and using the script can take place for people whose isolation is physical and bodily, thanks to sensory limitations in either seeing or a combination of seeing and hearing, without necessarily having access to an electrical transmitting and receiving device. It opens up a very low cost paradigm.
This is important, because there is no curriculum for imparting training of either Morse or Tap2Talk to either the blind or the deafblind. One needs to be developed, and this can be (and should be) done without becoming network-dependent.
However, using it is considerably or perhaps more beneficial when it becomes smartphone friendly. The need for a physical trainer might be eliminated entirely.
Such an app can use the left and right sides of the smartphone virtual keyboards for the long and short dash, and the lower and upper parts for pauses, either word, sentence or paragraph breaks.
It needs to be fully and easily adjustable for rate of input and output, so that a person can comfortably learn how to manage both input and output, the equivalent of writing and reading.
It needs to have levels of learning, so that a person can gradually improve both proficiency and vocabulary, since the well used Morse Code also has a universally accepted set of abbreviation codes for frequently used phrases.