Learn English – Term to describe relationship between one and single, two and double, etc.

We never say, I want an iced cream, or cream with ice, or cream which is iced, or anything else. We just say, I want ice cream. My point is when something is un-modifiable, that means it exists as a single unit. It may appear to have two words, but those two words, spoken separately, would have different meanings, so that particular word needs to be spoken in one breath to convey a particular meaning. So 'ice cream' is a one word. It has a distinct meaning.

Now consider, 'beautiful day.' Both Beautiful and Day have their own separate meanings and spoken together they convey a different meaning. Yet 'beautiful day' is not one word, because same can be expressed differently like, lovely day, pleasant day, awesome day, etc. More importantly, in 'beautiful day,' beautiful is an adjective. And if we start representing ice cream as having two words, would we say ice is an adjective here?

Websters gives the following definitions of unidirectional

1: involving, functioning, moving, or responsive in a single direction

2: not subject to change or reversal of direction

and bidirectional

involving, moving, or taking place in two usually opposite directions

I think those meet your needs. Specific to phone calls would be duplex for bidirectional. Half duplex communication is bidirectional but only one direction at a time.