What is the name of the literary technique/device used where the reader is only shown one side of a dialogue?

What is the literary device used when there is a dialogue but only one speaker's side is heard/shown to the reader/audience. And how can I write this in a way that it is clear that although this person is talking to someone else, and that person is responding, that the words present belong only to one speaker?

Solution 1:

Colloquially, this is the equivalent of hearing one side of a conversation. Sensibly, scriptwriter John August calls this a one-sided dialogue in a blog post titled, well, "One-sided dialogue."

Editor Deena Nataf calls it the same thing in a post of her own:

When writing phone conversations, does your one-sided dialogue give insight into the character of one or both participants or the relationship between them?

Heck, even scholars use the collocation. Michael von Albrecht, writing about ancient Roman forms of discourse, uses 'one-sided' dialogue as a suitable coinage to describe Seneca:

A distinction may be drawn between the didactic poem (s. above), the didactic monograph concentrating on one theme and using literary means to reach a wider audience (e.g. Cic. off.), the philosophical dialogue (several times in Cicero), the 'one-sided' dialogue, intended for self-instruction or as edifying reading (Seneca's writings) ...