Discussion Forum—A Way with Words, a fun radio show and podcast about language

I have a question as to what a practice is called. I'm not sure that it exists in English, but it does exist in Vietnamese.

In Vietnamese (southern dialect), the letter "d" is pronounced as an English "y" in both spoken conversation and when dictating to someone to write.

The letter combination "gi" has the same English "y" sound when spoken in conversation or when reading. BUT, to differentiate from "d" when telling some to write down a word, it is pronounced as an English "z".

So, the words da and gia sound the same when spoken in conversation (both are pronounced like "ya" in English). However, da is pronounced "ya" when telling someone to write the word down and gia is pronounced "za".

I'm trying to find out if there is a name for this practice of changing pronunciation based on whether something is for spoken use or written use.