Nothing like SMS
This is a specialist service for people who cannot use conventional speech telephones, to communicate 'in real time' with callers who are using ordinary speech-only telephones. A 'relay assistant' types what the hearing speaker says so that the deaf user can read it, &/or reads out to the hearing user what the deaf or unspeaking user types. Both users can type and read if that suits them better, in which case the nearest comparison is probably with a private IRC session.
It's a lifeline for those with serious speech or hearing difficulties.
It used to be necessary to use a fixed-line 'Minicom' or 'textphone' device to read or type, but the latest incarnation of the service allows the deaf user to use a smartphone or computer instead, which is a big step, and also uses 'text to speech' software instead of a human relay assistant for some purposes.